ANIMAL HISTOLOGY (from lo-rOg, a web or tissue, and AOyog, discourse) is the study of the minute structure of the tissues of animals. By a tissue is meant any part of an organism which has undergone special changes in structure in adaptation to the performance of special functions. These special changes are expressed by the general term "differentiation." In the lowest animal organisms, the whole of whose bodies are composed of the undifferentiated living substance termed " protoplasm," we find all its functions shared by every part of the organism. An amceba, for example, it is well known, is capable of finding, seizing, devouring, digesting, and assimilating food, has a special provision for collecting fluid and pumping it out of its body, respires by its whole surface, moves about apparently where it will, exhibits a sensibility to tactile impressions, and reacts in all probability to smell if not to sound and light, - in short, is capable of performing, although with the lowest possible amount of activity, almost every function which animals vastly higher in the scale of organization exhibit. But even in the amceba we cannot say certainly that there is no differentiation of its protoplasm. For a condensed portion - the nucleus - is set aside to initiate the reproductive function, and it is by means of the external and firmer layer (ectoplasm) that its movements are effected and its relations with the external world maintained, while the internal more fluid protoplasm (endoplasm) is concerned with the digestion of the food. Still there are simple organisms whose protoplasm is probably absolutely undifferentiated. On the other hand, there are other organisms which are also regarded as composed of simple protoplasm, and are constituted by a single cell, which nevertheless show a marked progress in the differentiation of portions of their substance apart altogether from the presence of a nucleus. Such differentiation in unicellular organisms generally takes the form of the production of a shell or " test," as in the Foraminifers and in Noctiluca, which subserves purely passive functions of sustentation or defence. It is not certain in such cases whether the structure thus produced is formed by the direct conversion of the protoplasm or by an exudation on the 'surface which subsequently hardens. But portions of the protoplasm may be set aside for the performance of active functions. We see this in its production in the form of locomotory organs, either temporary (pseudopodia) or permanent (cilia). But in neither of these can any actual change in the minute structure of the protoplasm be observed. A differentiation does, however, occur in one remarkable instance - the flagellum, namely, of the Noctilt/tic/cc (fig. 1), which exhibits as definite a transverse striation as does the cross-striated or voluntary muscular tissue of the higher animals, in which structural peculiarity it is impossible not to infer a relation to its contractile functions; and similarly, in the Yorticellidcc, there is a differentiation of the protoplasm of the rapidly contractile stalk.

Whereas in the more highly organized unicellular animals portions of the single cell are thus set aside for the performance of special functions, and modified in structure accordingly, in multicellular animals, on the other hand, we find whole cells and sets of cells set apart and differentiated. It is to such modifications in sets of cells in multicellular organisms, rather than in portions of the protoplasm of a unicellular organism, that " histological differentiation " is commonly restricted; and each such set of cells, destined for the performance of a special function, and modified accordingly in structure, is denominated a "tissue."

The animal tissues may be classed under the four heads of Epithelium, Connective Tissue, Muscular Tissue, and Nervous Tissue.' Of these four classes of tissue the epithelium is the most primitive and least altered. In the development of the Metazoa the numerous embryonic cells which result from the division of the single cell - the ovum - tend in nearly every case to arrange themselves as a single layer surrounding a central cavity (unilaminar condition of the blastoderm), (fig. 2, A).2 Presently a part of the wall of the hollow sphere becomes invaginated, so that, instead of a vesicle enclosed by but a single layer of cells, a cup (Gastrula, Haeckel), is produced (fig. 2, B), the wall of which is formed by two layers derived from the original single layer, and separated from one another by a narrow interval (which is all that remains of the original cavity of the vesicle) except at the orifice of the cup, where they are continuous (bilaminar condition of the blastoderm). At this part some cells become separated from oue or both of

The Epithelial Tissues. - Although, as we have seen (see p. 4, note 2), the layers of cells which are first formed are layers of epithelium, and, therefore, the epithelial tissues are the first to be produced, nevertheless we find that they undergo less modification in structure than any of the other three classes of tissue. As before said, they invariably consist merely of cells cemented together by an imperceptible amount of intercellular substance,1 and the cells themselves only show minor degrees of modification in shape and structure, at least as compared with the other tissues constituted mainly of cells, namely, the muscular and nervous.

Modifications in Shape of Epithelial Cells. - The cells of this tissue may be either elongated and set like palisades over the surface which they cover, in which case they are termed " columnar " (fig. 3), or they may be flattened out over the surface, and they then appear as thin "scales"; and every variation in shape is met with between these two extremes. In any case where they form a single layer, since the cells are set closely together, the mutual apposition of neighbouring cells produces a flattening of the opposed sides, so that, when the epithelium is looked at from the surface, the cells have a polygonal outline, and collectively present the appearance of a mosaic pavement (fig. 4). In

It frequently happens that the layer of epithelium which covers a surface is prolonged into depressions, which may be quite simple or may be ramified either slightly or in the most complex manner. The epithelial cells which lino such depressions may resemble those of the surface upon which the depression opens, or they may become more or less modified in size, shape, and other particulars, and constitute themselves into a distinct variety of epithelial tissue. Since depressions like those just mentioned are generally for the purpose of forming seine special secretion, and are termed ylands, and since this secretion is elaborated by the agency of the epithelial cells which line the gland, any such special variety of epithelium is termed a "glandular" or "secreting" epithelium.

_Modifications in ,S'tructure. - The modifications in structure which the cells of epithelial tissue undergo are comparatively slight. One of the most common is the conversion of the external layer of the protoplasm of the cell into a firm membrane, generally of a horny nature, but this membrane is seldom sharply marked off from the substance of the cell, as is the case with the cellulose membrane of the vegetable cell. It becomes formed, moreover, to a very different extent in different cells, according to the function which the particular epithelium has to perform ; where, for example, the epithelium is almost purely a protective covering, as in the stratified scaly epithelia, a considerable part, or even the whole thickness of many of the epithelial cells, is thus transformed ; but where, on the other hand, the cells have to play an active part in yielding a secretion to moisten the surface, or in protruding a portion of their protoplasm in the form of vibratile cilia to produce currents over the surface, or to move the organism through the water, we find little, if any, of such conversion of the superficial cell-substance. What little there may be is confined to the attached surfaces of the cell, or if there is any such covering on the free surface, it is penetrated by pores which allow of a communication between the protoplasm of the cell and the external medium.

Another common modification of structure which epithelial cells exhibit is the existence of vibratile cilia at the free surface (fig. 6). This, again, is especially frequent with cells of a columnar shape, but it may occur in any.

The cilia appear to be protrusions of the more active external protoplasm of the cell, which are in most cases incapable of being again withdrawn, and are in all probability modified in minute structure, although they are always so small that such modification, if it exists, escapes detection even with the employment of the highest powers of the microscope. At their base, however, the cilia are certainly continuous with the unaltered protoplasm of the cell. This may be seen even where the cilia are small and spring in a bunch from the free surface of the cell, but much better in those kinds of ciliated epithelium in which but a single large cilium is connected to each columnar cell (fig. 8).

Many epithelial cells, especially those of secreting glands, show a differentiation of their protoplasm in the form of fine strife or rods which pass from the attached border of the cell towards the free end (fig. 7). Cells thus modified are found in the ducts of the salivary glands, in the alveoli of the pancreas, and in the convoluted tubules of the kidney in Vertebrata.

One of the most remarkable modifications which epithelium cells exhibit is found in the organs of special sense. This is the presence of a fine filamentous process or processes springing from the free surface of the epithelium cells, and resembling in their appearance long cilia, but not spontaneously vibratile (fig. 9). Moreover, the cells in qiestion, which are generally of an elongated columnar form, commonly branch out at their detached end into fine processes which appear to become connected with nerve-fibres. Cells of this character occur even so low in the Meta:oa as the .3fedasce, in connexion with the nerve-epithelium to be afterwards mentioned. And, indeed, in many cases where cells of this character enter into the constitution of the sense organs, it is probably most consistent with their true nature to regard them as detached portions of nervous tissue, which also in every case is originally of an epithelial nature.

Modifications in the Cell Contents. - Another chief modification which the cells of an epithelial tissue may undergo consists in the accumulation within the cells of various chemical substances, which may be either taken in bodily as such, or may be formed in the cell from other substances which are supplied to it by the blood. The substances that are thus accumulated and formed within the cells of an epithelium are of very various nature, as, for example, the constituents of special secretions (fig. 10), mucin, pigment, fatty globules, uric acid, &c., Sze. These several substances are tolerably constant in an epithelium of the same kind - thus, mucin is a very frequent constituent of columnar epithelium, and in glands which have the same function in different animals, the same substances are found in the epithelium cells of the gland.

Exudation, from Epithelium Cells. Formation rf Cuticula.. Structures. - In many invertebrates the epithelium which covers the surface of the body, and sometimes also that which lines a part of the alimentary canal, forms an exudation which is generally soft at first, but may afterwards harden into a horny consistency, or may be rendered still harder and at the same time more brittle by impregnation with earthy salts. Any such structure is termed a cuticular formation. It may be composed of a single thin layer, or a number of layers may be superimposed, so that a " shell " of considerable thickness is thus formed. - The chitinous or calcareous covering which forms the exoskeleton in many molluscs, arthropods, annelids, and Hydrozoa is of this nature. On the other hand, the firm skeletons of sponges, Actinozoa, and Echinodermata are formed by deposition in the connective tissue.

The Connective Tissues. - The connective tissues are characterized by the great development of intercellular substance in comparison with the cells ; indeed in those animals in which connective tissue may first of all be said to appear, there is an entire absence of cellular elements properly belonging to the tissue. This is the case in many of the Codenterata, in which the connective tissue is represented merely by a layer, more or less thick, of hyaline substance, which undoubtedly performs a sustentacular function, in addition to connecting together the epithelial layers of the ectoderm and entoderm.

The intercellular or ground substance almost invariably takes a prominent part in the formation of connective tissue. It is of a semi-fluid nature, and often contains in addition to albumen a certain amount of mucin. In most cases the cells of the connective tissue separate themselves from the primary layers before the formation of this ground substance ; indeed the mesoderm is at first chiefly formed of these cells. The stages of development are as follows. The mesodermic cells, which are at first in apposition, become separated from one another by the accumulation of intercellular substance, but at the same time maintain a connexion with one another throughout the tissue by their branching cell-processes (see fig. 2, C, c). Presently, in the production of ordinary connective tissue, fibres of two kinds make their appearance in the intercellular substance, and to all appearance independent of the cells.

Those of the one kind (fig. 11, A) are highly elastic and refracting, not easily affected by reagents, stain deeply with magenta, run singly, always branch, and become united with neighbouring fibres so as to form a network throughout the tissue ; those of the other kind (fig. 11, B) are excessively fine and indistinct, never run singly but always in bundles, and generally with a wavy course, are readily affected by reagents, and, in vertebrates, yield gelatin on boiling. In the various kinds of connective tissue the relative proportion of these two kinds of fibres to one another and to the cellular elements of the tissue varies. Thus in the so-called elastic tissue of the Vertebrate the elastic fibres greatly preponderate ; in tendinous tissue, on the other hand, they are scarcely to be found, and the ground is almost wholly occupied by the white fibres. It may happen that the intercellular substance is so completely occupied by the fibres as to be entirely obscured, but its presence may be always recognized in consequence of the property which it possesses of reducing silver from its salts when exposed to the light. In certain cases the intercellular substance becomes hardened by the deposit within it either of a substance termed chondrin, which confers upon it the well-known toughness and elasticity of cartilage, or by a deposit of earthy salts imparting to it the firmness of bone. These several changes in the intercellular substance are accompanied by special modifications in the form and relations of the cells (by whose agency they are in all probability effected). In comparatively rare cases the intercellular substance which is found occupying the meshes of the network formed by the branched cells of the developing connective tissue may disappear entirely, and the meshes may be occupied either by blood or by the lymph or plasma of the blood (spleen and lymphatic glands of vertebrates).

It frequently happens that the connective tissue presents the consistence of jelly, and this is generally ascribed to the characters of the intercellular substance. It may, however, be due in many cases to the entanglement of fluid in the meshes of the fibres, and not to a gelatinization of the ground substance. This is shown by the fact that the fluid may be drained from out the meshes by means of filter paper. And the possibility of the formation of a jelly in this manner is evidenced in the coagulation of lymph, where the apparently solid gelatinous clot is a tangled meshwork of fine filaments enclosing fluid.

The connective tissues of invertebrates are, on the whole, similar to those of the vertebrate ; at the same time it must be admitted that there are not unimportant differences in chemical constitution, such as the absence of a substance yielding gelatin, and the absence for the most part of mucin, both of which are characteristic constituents of vertebrate connective tissue. On the other hand the anatomical characters of the elements, both cells and fibres, are in most cases sufficiently well marked to be recognizable.

In the sponges the bulk of the animal is made up of a jelly which, when examined under the microscope, is found to consist of large branched cells (fig. 12) connected together by their processes into a network. The meshes of this are occupied by clear intercellular substance within which the calcareous or horny matter which forms the skeleton is deposited.

When the development of the sponge is traced it is found that the first part of the tissue to be seen is the clear intermediate substance, and the skeletal spicules begin to appear in this before the cellular elements. These wander subsequently into it from one of the primary layers. There can be no doubt that this jelly-like tissue of the sponge represents a primitive form of connective tissue, although, so far as has at present been ascertained, no fibres are developed in it.

In the Ccelenterata, as in the sponges, the connective tissue makes its first appearance in the form of a clear intermediate substance, which may be so small in amount as to be almost imperceptible, or so large in amount as to form the main bulk of the organism. In the former case, as in the developing sponge, there is an entire absence of both fibres and cells, whereas in the latter case both kinds of elements are found. The fibres are the more con stant, and are of time elastic kind (fig. 13) ; they have for the most part a direction across the thickness of the tissue stretching from entoderm to ectoderm, branching and uniting with their neighbours to form the characteristic network which, enclosing watery fluid in its meshes, produces the jelly-like consistence of the tissue. Fibres of the white variety are also found as low down in the Metazoa as the Cdelenterata. In some of the acraspedote Medusa? they occur in the form of bundles of indistinct wavy fibres situated near the surface of the jelly, and in the Actinice similar fibres are found forming membranes which bear a strong resemblance to some of the forms of membranous connective tissue of the Vertebrata (fig. 14). As before

We see then that the cells, the intercellular substance, the white fibres, and the elastic fibres of the vertebrate connective tissue are represented in these low forms of the Metazoa in a perfectly recognizable manner. It is not surprising, therefore, to find in all the higher classes of tho Invertebrata that similar elements characterize the connective tissue, although there are undoubtedly certain modifications and exceptions. The most noteworthy modifications occur in the chemical constitution of the ground substance and of the fibres. Thus, as before mentioned, there is for the most part an absence of the gelatin-yielding substance of the vertebrate connective tissue. On the other hand, the intercellular substance may become infiltrated with chemical principles unknown in vertebrate histology, as in the tunic of the Tunicata, where cellulose is found.1 There are modifications also in the appearance of the connective tissue fibres which are often accompanied by modifications in the chemical constitution. For example, in the Arthropoda the tissue often undergoes extensive chitinization, and the fibres in it present a straight, stiff appearance, very unlike the soft, wavy look which is exhibited by the fibrous tissue of the Vertebrate.

Although the ramified cell may be looked upon as on the whole the most characteristic form of cell met with in connective tissue, and although this is the first modification in shape which the rounded embryonic cells of the developing vertebrate connective tissue take on, nevertheless it gives place in many parts both in invertebrates as well as vertebrates to other forms. One of the commonest of these is the fiat cell, and we almost invariably find cells of this description lying on or in connective tissue membranes, and lining cavities which may have become formed in the connective tissue. In the latter case the flat cells may be and most commonly are spread over the whole inner surface of the cavity which they line, and assume the appearance of a pavement epithelium. Such cells, which are termed epithelioid (or by some endothelial), are found lining the body cavity and the vascular canals and heart (where these exist) of all invertebrates just as they do the similar cavities and canals in vertebrates, and they are derived like the rest of the cells of the connective tissue from the mesoderm, and therefore only indirectly from the primary blastodermie layers. But in the holothurians, and some other animals, the cells in question are derived directly from the entoderm.

In the Mollusca (fig 15) a peculiar type of connective tissue cell makes its appearance in addition to the rounded, the ramified, and the flattened forms. This takes the shape of a large clear, vesicular, donble-contoured eell-body (v) with a relatively small nucleus. Cells of this character are in some cases only to be found scattered here and there in the tissues, but in others they are closely collected masses, and by their aggregation confer an almost cartilaginous consistency upon the tissue. This is not due, however, to the accumulation of ehondrin - the chemical principle of the cartilage of vertebrates - in intercellular substance. But a true cartilage is met with in some of the higher molluscs (Cephalopoda), in which there is a considerable amount of intercellular substance, and the only difference, as compared with ordinary cartilage of most vertebrates, is that the cells are much ramified (as in some fishes).

Bone, or osseous connective tissue, as the word is understood in vertebrate histology, is not met with anywhere amongst the Invertebrata, and this is less to be wondered at since it does not make its appearance even in some of the lowest of the Vertebrata. But hard structures of various kinds serve to supply the place of bone as a sustentacular tissue, and these may be developed either within the connective tissue, so as to form an internal hard skeleton, or on the exterior of the body, so as to form an external skeleton or shell. When external the shell is an epithelial structure, or at least is produced by the formative activity of the epithelium which covers the surface of the body. An internal hard skeleton may either coexist with an external, or the one may be found to the exclusion of the other.

In the coelenterates the internal skeleton when found is generally deposited in the jelly-like intercellular substance in the form of separate spicules, which subsequently are cemented together by a further deposit of calcareous matter into a continuous skeleton. In the sponges the calcareous spicules often project fromthe jelly into the externalmedium, but it is probable that they are covered by an extension of the superficial flattened cells which they seem to pierce; the separate calcareous spicules are in some cases united by calcareous matter, in others by horny substance, or the spicules may be altogether absent and the horny framework constitute the whole skeleton. There is no evidence to show that these calcareous and horny deposits are formed by the direct agency of the cells of the jelly-like tissue. On the contrary, the fact that the spicules make their appearance in the jelly-like substance which accumulates between the two primary layers before there is any trace of cells to be seen in it is a fact pointing in the opposite direction.

In the echinids amongst .Echinodermata the shell is formed by a dense deposit of calcareous substance ('fig. 16) in the fibrous connective tissue of the integument, but is not of the nature of bone, as has been sometimes supposed. In other echinoderms the deposition is more scanty, and in some (Holothurict) it may merely take the form of isolated spicules, which often present curious shapes.

The Muscular Tissues of Animals. - In the "Vertebrata three kinds of muscular tissue are met with - the plain or involuntary, the cross-striped or voluntary, and the cardiac or heart muscle. Undoubtedly the last-named is to be regarded as a transitional form between the other two, for it combines some of the characters of each. This is especially well seen in the lower vertebrates, in which the muscular fibres of the heart (fig. 17) consist of long, tapering, uninuclear cells, in form resembling the plain contractile fibre-cells,1 but differing from these and resembling the multinucleated voluntary muscular fibres in exhibiting distinct transverse striations, Although these three kinds of muscular tissue thus differ from one another in this respect, they agree iu one important character. Whether transversely striated or not, they all exhibit a distinct longitudinal striation of their substance, which is probably indicatory of a polarity which the protoplasm of the cell has assumed at the same time with the faculty of becoming rapidly shortened in the direction of its length and coincidently with the loss of the power of contracting in other directions. Moreover, this longitudinal striation is generally associated with the property of double refraction, which is exhibited to a marked degree by all kinds of muscular tissue.

The voluntary muscular fibres are those in which the protoplasm of the original cell has undergone most differentiation. If we trace their development we find that they originate from mesodermic cells which become elongated in one direction, the nucleus undergoing a corresponding change in shape, and soon becoming multiplied ; we next find the external layer of the protoplasm becoming altered and converted into muscular substance, which exhibits from the first both a longitudinal and a transverse striation. The change in question gradually extends inwards, so as to involve more and more of the protoplasm. Up to this time we can distinguish (fig. 18) in the muscular fibre a medullary part composed of unaltered protoplasm, with nuclei, and a cortical part composed of differentiated muscle-substance. Subsequently the nuclei leave their central situation, and either become scattered through the muscular substance or come to lie entirely at the surface. There is always a little of the unaltered protoplasm to be found with each nucleus.

In the plain muscular fibres, and in the cardiac muscular fibres, the nucleus does not multiply, and it maintains its central situation. The differentiation of the cell-protoplasm into muscle-substance begins at the periphery and extends towards the centre in the cells which constitute the heart-muscle as in the voluntary muscle, and it is probable that the same is the case in the plain muscular cells.

The muscular fibres of the Invertebrata very closely resemble those of vertebrates. In most cases the differentiation of the muscular substance is not so complete as in the voluntary muscles of vertebrates and especially of mammals, but there is a striking exception in the Arthropoda, and especially in insects, where in conformity with the greater muscular activity they possess we find far better marked structural features. On this account the muscles of insects have been especially carefully studied with a view to the elucidation of the structure of muscle generally.

With a sufficiently high power a voluntary muscular fibre of an insect (fig. 19) is seen to be composed of an external structnreless membrane, - the sarcolemma, --a central strand of nucleated protoplasm, and a semi-fluid substance - the proper muscular substance - lying between these and forming almost the whole of the fibre. This proper muscular substance is composed of a clear doubly-refracting material, in which are embedded a number of minute rod-shaped particles, which arc so arranged side by side and end to end as to cause the muscular substance to present both a transversely and a longitudinally striated appearance. Sometimes (as in the fibre shown in the figure) the substance of each muscle rod is partly collected into a swelling or knob at either end, and these knobs so act singly and collectively upon the light which passes through the muscular substance as to cause a brighter appearance in their neighbourhood. In this way bright bands seem to cross the muscular substance alternating with the dimmer intermediate portions, and the appearance of transverse striation is much intensified. This is still more the case when the muscle contracts, for the contraction is accompanied by an accumulation of the substance of the rods towards their ends, and an apparent blending of these into a dark transverse band or rather a series of dark transverse bands, which, reflecting from their surfaces the light which is passing through the muscle, cause the whole of the substance between them to appear much brighter than they are themselves. There are other muscular fibres in the insect which present an entirely different appearance. In these (fig. 20) the fibres, which , are very fine, are wholly made up of alter nating bands of dark and light substance.

They are far less like the voluntary muscular fibres of mammals than are the others, and there are no rod-like structures to be seen in them.

We find muscular tissue, like the other tissues, appearing already in the lowest of the Macaw. In sponges the orifices of the water canals are in many cases capable of being closed partially .or wholly when the organism is irritated. The researches of F. E. Schulze have shown that these orifices are encircled by long fusiform cells which appear to be modifications of some of the ordinary ramified cells of the jelly-like tissue. The substance of these cells seems to be undifferentiated, and it cannot be conclusively affirmed that they are of muscular nature, but at least they seem to subserve the function of muscular tissue. But in the very next division - the Hydroutedusce - the muscular cells are already so much differentiated as to exhibit both longitudinal and transverse striation. Thus many of the cells which form the muscular layer of the sub-umbrella of the Medusce are long fusiform cells (fig 21, A) with an elongated nucleus in the centre, and gradually tapering ends, and their substance is striated, as just remarked, both transversely and longitudinally. Sometimes there is a considerable amount of unaltered protoplasm in the middle of the fibre around the nucleus (fig 21, B), and this nucleated protoplasm may then project between the epithelial cells of the ectoderm. In every case the muscular fibres are in close contiguity with the attached ends of the ectodermal epithelium, and are with justice reckoned as a part of the ectoderm. In the higher ccolenterates the muscular tissue tends to lose its connexion with the ectoderm and to become embedded in the jelly-like mesoderm, but the connexion is not wholly lost in any. In Hydra, on the other hand, the muscular tissue is represented only by simple longitudinal fibres, which are either direct prolongations of the tapering ends of some of the ectoderm cells (Kleinenberg) or are embedded in the enlarged attached end of the cells (Kolliker, Korotneff). In other invertebrates the muscular tissue is nearly always in the form of long cylindrical or flattened, tapering or uniform, uninucleated, longitudinally-striated fibres, which may possess a membrane, and a central strand of undifferentiated protoplasm (fig. 22, B). In some cases a transverse striation may be detected (fig. 22, A), but more commonly the muscular fibres, especially in echinoderms, worms, and molluscs, exhibit a peculiar double oblique striation (fig. 23), so that an appearance of intercrossing lines is thereby produced. The obliquely striated fibres seem to take the place, in many of these comparatively sluggish aninials, of the more active, transversely striated tissues. With the exception of the appearance mentioned, they resemble the plain muscular fibres in structure, but they are capable of more energetic contraction than the latter.

The Nervous Tissues of Auintals.--The nervous tissue of vertebrates is composed firstly of cells - the nerve-cells or ganglioncells, - and secondly of nerve-fibres. Most of the nerve-fibres possess a sheath formed of nucleated cells wrapped around the fibre,.

and in this sheath a peculiar white fatty so-called medullary substance is accumulated in some fibres, so that they are distinguished from the others as the white or medullated fibres. There is reason to believe that every nerve-fibre is connected with at least one nerve-cell, and conversely, that every nerve-cell is connected directly or indirectly with one or more nerve-fibres. Nerve-cells are generally comparatively large solid-looking corpuscles, with a relatively large nucleus and nucleolus, and every developed nerve-cell has either one or two or a greater number of processes, which may or may not he ramified. It is certain that from many nerve-cells one process of the cell passes into and becomes a nerve-fibre. 1 Nerve-cells are always traversed by ex quisitely fine fibrils, - nerve-fibrils, - and these pass out from the cell into its processes. Apart from any sheath which it may possess, a nerve-fibro is composed of one or more nerve-fibrils, which are embedded in a soft interfibrillar substance. The nervous tissue of vertebrates is developed from that part of the ectoderm which occupies the middle of the dorsal surface of the embryo. In the bird and mammal tho epithelial cells in this situation become cut off from the general ectoderm by the formation of a groove which subse quently closes over and forms a canal - the neural canal. The innermost ectoderm cells (fig. 21, If) which form the wall of this canal acquire cilia at the end which is turned towards the cavity, while the other end of each cell is prolonged into branching processes which collectively form a network amongst the deeper lying cells of the wall. The latter multiply considerably, and moreover groups of them grow out from the sides of the neural canal as the roots of the nerves. The nerve-fibres themselves seem to be formed either by the outgrowth of undivided processes from these cells of the neural canal, or by the junction of one elongated cell with others. At any rate the fibres are to be looked upon as outgrowths or prolongations of nerve-cells. But some of the outgrowths of the nerve-cells, instead of passing into nerve-fibres, become ramified, and eventually break up into fine twigs, each of which is occupied by a nerve-fibril, and these form by their interlacement a network which joins that of the branched processes of the ciliated epithelium.

With the exception of the formation of the medullary substance in the sheath, the nervous tissue of the invertebrate Metazoa agrees precisely, so far as the minute structure is concerned, with that of vertebrates. The lowest forms in which nervous structures have been found are the ilfedusee. In these the tissue exhibits itself under two modifications. The first of these is a so-called nerve epithelium (fig. 24, A), consisting of a portion of the ciliated ectoderm, the cells of which are prolonged at their attached ends into fine ramified fibres which interlace with one another and form a network of nerve-fibrils underneath the epithelium. This form seems to correspond with the ciliated epithelium of the vertebrate neural canal. The second modification occurs in certain cells of the ectoderm, which have become sunken singly here and there below the general epithelium of the surface, and between it and the muscular layer of the sub-umbrella. These cells become enlarged, and their nucleus takes on the characteristic appearance of the nucleus of a nerve-cell. Then generally from opposite ends of the cell (fig. 25) two processes grow out into long fibres, which exhibit all the features of the nerve-fibres of higher animals, and may even possess a nucleated sheath. These fibres, which may be branched or unbranched, seem to be applied to the substance of the muscular fibres, and in all probability serve to convey impulses to the muscle. There can be no doubt of the correspondence of these cells and fibres with the nerve-cells and nerve-fibre prolongations of the Vertebrata. In other invertebrates the nervous tissue is not only more localized than in the axlenterata, but the original ectodermal epithelium cells from which it is derived become much more extensively developed into well-characterized nerve-cells and nerve-fibres, and tend moreover to be completely separated from the rest of the ectoderm and embedded in the mesoderm. But they are never originally developed in common with connective-tissue cells, as are the cells which form the muscular tissue. (E. A. s.) H. VEGETABLE HISTOLOGY.

By Vegetable Histology is meant the study by means of the microscope of the texture, web, or tissue of which plants are composed. It may be considered as synonymous with the minute anatomy of plants, and embraces the study of alt those points of structure and development requiring the use of the microscope for their elucidation. Histology is, therefore, a modern science of observation and experiment, and it dates its origin from the time when magnifying glasses were first applied to the scrutiny of the organs of plants. All advances in histology have been preceded by some important improvement either in the construction of lenses and microscopes, or by the invention of some new method of research and application of new reagents. In order to prosecute the study of vegetable histology, it is necessary to understand thoroughly the construction and use of the microscope, to be able to manipulate well and dexterously employ the various cutting and other instruments required, and, lastly, to be able to use the numerous reagents now so important in assisting to unravel the more difficult tissues.

Xecture of the Vegetable Cell. - If a small portion of the contents of the fertilized embryo-sac of the Phase°lus multi-ferns (scarlet-runner, or French bean) be examined in a drop of water on a slide, it is seen to consist of protoplasm with a number of small free cells, in different stages of development, floating in it. These cells consist of little rounded masses of protoplasm with a single contour line ; they have the protoplasm more or less granular ; and each contains a rounded solid body, the nucleus, usually with a small spot, the nucleolus. Other cells in the preparation have a distinct wall with a double contour line, these being older and more fully developed. In examining the cells it is usually best not to employ pure water, but to use instead a solution of sugar or gum (1 part to 50 or 100 of water). Strasburger recommends, for examining the contents of the embryo-sac in phanerogams, a 3 per cent. solution of sugar, to which is added on the slide one drop of a 1 per cent. solution of osmic acid. Absolute alcohol may also be used for fixing the protoplasm in a nearly unaltered state. A longitudinal section of the growing end of the root of Fritillariaimperialis will exhibit the different stages of development of tissue cells. Near the apex the cells are more or less hexagonal in shape, and have a marked wall with a more or less distinct double contour. Inside the cell-wall, and in close contact with it, is the protoplasm, a densely granular soft inelastic mass, consisting of a mixture of albuminoids, and having in the centre a round and relatively very large solid nucleus, with one or two nucleoli. In both Phaseolus and &itillaria, as the cell enlarges, clear spaces, called vacuoles, but filled with cell-sap, that is, water with substances in solution, appear in the protoplasm of the cell. In some algm contractile vacuoles are met with. The ordinary vacuoles rapidly increase in number and enlarge, separating the protoplasm into two parts - one in close contact with the wall of the cell, the other' forming strings of varying size and thickness separating the vacuoles. Presently the vacuoles all coalesce and form a central cell-sap cavity, the protoplasm forming a completely closed sac inside the cell-wall. The nucleus remains imbedded in the protoplasm, and is pushed to one side, appearing as if in contact with the wall. The vacuoled condition of the protoplasm may be considered as representing the cell at its state of greatest activity : the central cell-sap cavity is usually seen in tissue-cells, as in Fritillaries, and may he taken to indicate a condition of diminished activity. Further changes take place in tissue cells. The protoplasm with its nucleus may disappear and the cell-sap remain, or even the cell-sap itself may disappear sooner or later, and the dry cell-walls, as those of cork, be left. The conditions here described in Fritillaries may be taken as typical of all young tissue cells.

The protoplasm is the essential part of the cell, and by it all the other parts are formed, as well as all the substances, such as chlorophyll and starch, that are contained in cells. When the cell contains protoplasm it can grow, multiply, and elaborate new chemical compounds ; when the protoplasm disappears it ceases to perform any of these functions, and passively acts as a protection to deeper cells, or permits certain physical processes to take place, as the transport of water through the walls. The substance of the protoplasm seems to consist of a mixture of various albuminoids, and probably of other nitrogenous compounds. It is a more or less granular, soft, inelastic substance, never a true fluid, but varying in consistence in accordance with the quantity of water it contains.

The chemical reactions of protoplasm are those of albumen. It contracts when substances are applied to it which remove some of the water, as glycerin and alcohol. It contracts when heat is applied, a temperature of between 50° and 60° C. completely altering the texture of protoplasm containing a normal quantity of water. Where the protoplasm contains little water and is very dense, as in some seeds, a higher temperature -produces little or no change. A violet colour is given to the protoplasm of young cells by the application first of a concentrated solution of copper sulphate, next washing the preparation carefully to remove all free traces of the copper solution and then applying a solution of caustic potash. Iodine gives a brown colour, sugar solution and sulphuric acid a red; and dilute caustic potash dissolves protoplasm or renders it perfectly transparent. Carmine and other colouring matters do not colour living protoplasm, but impart a brilliant stain to it when dead.

Protoplasm is usually separable into two parts, an inner portion, endoplasm, more or less granular, and an outer more dense layer, the ectoplasm or primordial utricle, which is quite free from granules. A similar layer surrounds the protoplasm of the nucleus.

The living protoplasm exhibits movements either when inside a cell-wall or when the protoplasm is free and in the condition of a wall-less or primordial cell. The constant changes in protoplasm must be always accompanied by movements, but these are usually too small to be visible, and it is only in a few cases that the amplitude of the movements renders . them visible, The movements are of four kinds, and are distinguished as rotation, circulation, amceboid, and ciliary.

The first is the movement of rotation, as in Vallisneria and Anacharis, where the whole protoplasmic sac rotates in the interior of the cell. The second is circulation, where portions only of the protoplasm move as indicated by the circulation of the granules hither and thither in the mass, as in the cells of the hairs of Tradeseantia and the stinging hairs of the nettle. In the third or amceboid movement whole masses of protoplasm not enclosed in walls change their form and position like the ameba or white blood corpuscle. These movements have been noticed in the ammboid and plasmodium stage of the Myxomycctes or gelatinous fungi. Lastly, a movement of small masses of protoplasm destitute of walls, and having parts of the ectoplasm prolonged to form one, two, or more vibratile cilia is not unfrequent iu the zoospores, swarmspores, and spermatozoids of eryptogamic plants. All these movements are de- pendent on and much influenced by varying external conditions, as light, heat, presence or absence of oxygen, &c.

The cell-wall is a thin, elastic, transparent and colourless membrane, destitute of visible openings (except in some cells of Sphagnum and in bordered pits), but easily permeated by water and gas. It consists of the carbohydrate cellulose (0,111005), isomeric with starch, and in young cells it is present in an almost pure state. During the growth of the wall various chemical and physical changes occur in it.

The increase in the size of the cell is rarely quite regular or general, except in free cells as pollen grains and spores ; usually the growth is more or less limited to definite parts of the wall, and the increase in size is accompanied by a marked change in form. Intercalar growth at a ring-like zone on the cell-wall is seen in the genus CEdogorcium, while growth at the apex of the cell is not uncommon in many unicellular algm and in hairs, as well as in the peculiar cells (the hyphm) of fungi.

Growth at several points on the surface of a cell gives rise to the stellate forms seen in the pith of Anew, and a similar hut more limited growth is the cause of the "tyloses," or cellular filling-up of vessels seen in many stems, vine, &e. The growth of the cell-wall in thickness may be general or local. Usually it is local, and is either internal (centripetal) or external (centrifugal). Local thickening gives rise to the. production of peculiar markings depending on the different optical effects produced by the thickened and unthickened parts. Pitted markings are very common, rounded or variously shaped portions of the wall being left unthiekened, while the form of the pits, and their special arrangement, either irregularly scattered or spirally placed, give a characteristic appearance to the walls of the cells. Pits are often elongated, and when very much elongated, and extending the whole width of the cell, form scalariform markings, as seen in ferns. When pits are very narrow, cylindrical, deep, and branching, they form canals. Bordered pits, in which the pit is surrounded by a border, occur in the pines. In other cells the thickening assumes the appearance of rings, spirals, or reticulations, which sometimes heroine detached from the walls. In some instances, as in the wood of the lime and yew, two kinds of marking occur in one cell. Peculiar modifications of internal thickening are'seen in the root-hairs of Miirchantia, in the cells with cys tolithes in the leaf of the India rubber, and in the pith of Bicinsss, &c. External thickening is seen on the surface (cuticle of the epidermis) of the plant or ou free cells, as pollen grains, spores, &c., and produces peculiar and characteristic markings in various plants. By the alternation of more and less watery layers the cell-wall becomes marked by concentric lines or strim, as if the wall was built up of layers or strata formed one inside the other, which, however, is not the case. A longitudinal striation assuming a ring-like or spiral direction is also met with on the walls of many wood and bast cells, and is, like the stratification just mentioned, due to alternations of more and less watery layers in the cell-wall. The inner layer in the interior of the cell and next thelcontents is always a dense layer rich in cellulose and with little water, a fact at once negativing the incrustation theory. Stratification can be readily seen in transverse sections of the bast fibres in the leaf of Hoya, or the bast of the stems of many Aselepiaclaeco; and the longitudinal striation may be seen in the same fibres when dried, or in the dry wood cells of many conifers, as in Pinus sylrestris.

The walls of young cells consist almost exclusively of pure cellulose, which is coloured blue by Schultz's solution' or by iodine and sulphuric acid, and is dissolved by strong sulphuric acid and by ammoniacal solution of cupric oxide. Iodine solution alone gives no reaction, or more generally a brown tint ; rarely the wall gives a blue reaction, as in the asci of some lichens or in the cells of the cotyledons of Tamarindus indica. The cell-walls of most fungi do not give a blue reaction with iodine and sulphuric acid, forming the modification generally known as fungus cellulose. During growth changes occur in the nature of the wall, different• strata often having different chemical and physical properties. The three most important changes in the cell-wall are (1) the suberous or corky change, the cell-wall wholly or partially becoming cuticularizecl or converted into cork ; (2) the ligneous or woody change, the walls being converted into wood ; and (3) the gelatinous change, as seen in many algae, where the cell-wall swells up enormously by the imbibition of water, and assumes a clear gelatinous appearance. These changes may occur separately, the whole wall being more or less completely changed ; or a part remains composed of cellulose ; or, in other cases, two or more of these chatiges may coexist in the same cell-wall.

The following reagents are useful in distinguishing the different changes. Schultz's solution gives a blue with starch and cellulose, and a yellow-brown with wood and cork. If the cork-cells are previously boiled in caustic potash, and the wood cells touched with nitric acid, the blue reaction may be got with sulphuric acid and iodine. Sulphuric acid dissolves wood cells, but does not touch cork cells. Ammoniacal solution of cupric oxide does not dissolve cork, causes wood to swell up and to become blue, and deeply colours mucilaginous walls. Boiling caustic potash ultimately dissolves cork. Cold caustic potash at first causes it to swell np and become yellow, and when slowly heated the colour deepens and the texture becomes granular.2 Chlorate of potash and nitric acid (Schultz's macerating fluid) ultimately dissolves cork, like caustic potash, hut does not affect wood. When cork-cells are slowly warmed in this mixture the walls of the cork-cells become very distinct, the other cells being very transparent, and if washed and treated with alcohol and then with ether, they become perfectly transparent. Chromic acid renders cork distinct by rendering other tissues transparent. Bichromate of potash dissolves cork. The following reactions are given by Zacharias for cell-walls which are coloured brown by Schultz's solution in the rhizome of Acorns Ca2mmu.s.3 Sulphate of aniline and hydrochlorate of aniline, even when the cells are previously treated with hydrochloric acid, give no reaction, but colour the walls of vessels of a golden yellow. An aqueous solution of aniline blue gives no reaction, while an alcoholic solution of aniline red colours the walls of vessels and oil-glands. The red colour is best seen when thin slices of the rhizome are placed for a few days in the solution, and then dried and observed under water. The addition of caustic potash causes the colour to disappear, but it reappears on washing away the potash. The walls of vessels become coloured blue-violet by addition of hydrochlorate of phenole, and also when use is made of alcoholic solution of cherry-wood and concentrated hydrochloric acid (llohnel's xylophilin reaction).

Mineral matters are often deposited in cell-walls. Calcium carbonate occurs rarely, calcium oxalate frequently, and silica is the commonest of all. Calcium carbonate forms the cystolithes of Ficus and of the Acanthacece, crystals or masses of crystals imbedded in the cell-wall but projecting into the cavity although surrounded by the substance of the wall. In corallines and many algae, as also in the Charas, carbonate of lime is abundant in the cell-walls. Calcium oxalate, crystals occur in the cell-walls of many plants, in other cases forming small granules. The crystals of calcium carbonate are soluble in acetic acid, while those of oxalate are not, although soluble hi dilute nitric and hydrochloric acids. Silica is abundant in the Diatomacew and also in the cells of many of the higher plants (Equisetum, grasses, beech, &c.).

Products of desorganization or degradation of the cell.wall occur in the form of gum, gum-resins, or resins, examples of which may be seen in the cherry, gum-arabic, gumtragacanth, myrrh, &c. Gum-arabic consists of arabin, gum-tragacanth of bassorin, and cherry-gum is a mixture of the two. These substances, when formed, are apparently of no further use to the plant, and are produced by the destruction or desorganization of the cell-walls, as portions of the cell-wall can be distinctly traced when gum-tragacanth is examined microscopically.

Cell-walls, as those of the wood of Conifers, bast-cells, - and cells of ivory nut, and starch granules, are found when examined by polarized light to be doubly refracting. By an elaborate series of researches Niigeli concluded that these structures were made up of crystalline doubly refracting particles or naicell, each consisting of numerous atoms and impermeable by water, although each of the micellte is surrounded by a thinner or thicker layer of water. The water may increase or diminish within certain limits without destroying the structure ; or under certain conditions as by the application of certain reagents (strong acids and alkalies, 'ammoniacal solution of cupric oxide), the texture can be destroyed by the swelling up of the part. The water between the micellee may be removed by drying, when the micellte themselves come into contact, as the presence of air would destroy the transparency of the membrane. This peculiar molecular composition of the wall at once explains the striation and stratification observed in it, and also enables us to understand growth and nutrition by the intussusception of new particles in the water space between the mice11.1 Certain substances are formed by the protoplasm and separate from it in the form of granules or crystal-like bodies. The most important of these substances are chlorophyll and starch, the less important are aleurone grains and crystalloids.

Chlorophyll or leaf-green is the green colouring matter of plants, and is met with most frequently in the leaves and young stems. The colouring matter is always united with the protoplasm, usually to definite rounded masses, the chlorophyll granules or corpuscles, readily distinguishable from the general protoplasmic mass of the cell in which they are imbedded. Chlorophyll granules never occur separate from the protoplasm of the cell. In a few instances the whole of the protoplasmic mass, with the exception of the ectoplasm, is uniformly coloured green as in P/curococcus and other low algfe ; while in other plants the protoplasmic base for the colouring matter is star-like (Zugatema), in plates or lamellae (Clostcrigent and Me-socorpus), or spiral, as in Spirogyra. The chlorophyll grains of the vast majority of plants are rounded corpuscles of varying size with a slightly denser external layer, and frequently containing vacuoles or small starch granules. They grow in size and divide, the grain elongating and being cut into two by the formation of a gradually deepening circular groove. These changes may be seen in the prothallus of a fern or the leaf of a mos;5. The granules are produced by the aogregation of protoplasmic particles, so as to form a sharply- defined spherical mass. At first these are colourless or of a yellow tinge, and become green by the formation of the colouring matter, the Adorophyll, when exposed to the light, as it is only in a few rare cases, as in the cotyledons of pines and in ferns, that the colouring matter is formed independently of light.

The colouring matter can be removed by means of alcohol, ether, benzole, chloroform, and other solvents, the protoplasmic mass remaining behind unchanged in size and appearance, except in so far that it is now colourless. The solution thus obtained is of a dark green colour by transmitted light, and blood-red by reflected light. Its spectrum shows seven absorption bands, the strongest being between the lines a and e of the solar spectrum. Alany modifications of chlorophyll exist in plants, and it also undergoes changes in colour during the ripening of fruits or in the corollas of certain flowers. The chief modifications are - etiolin, in blanched parts of plants ; anthoxanthin, in yellow granules of many flowers ; xanthophyll, yellow granules in leaves in autumn ; • the green colour- ino. matter of red sea-weeds ; phycoerythriu, the red colouring matter of red sea-weeds ; the phycochrome of nostoc, &c.; and the brown colour of diatoms and fncoids.

Starch occurs in granules of varying size and form, and during the growth of the granule it is always in relation to the protoplasm of the cell. The granules are oval, lenticular, polyhedral, or bone-shaped, as may be seen in the potato, wheat, and maize, and in the milk-sap of certain exotic Eupho•bia8 respectively. Each grain usually exhibits a central or lateral spot, the Minim and a series of concentric strhe, caused like the striation and stratification of the cell-wall by the alternation of more and less watery layers. Sometimes the starch granule has two or more hila, the compound grains, which often separate into their several parts.

Starch has the same chemical composition as cellulose, Cdi,„0„ and differs from cellulose in being coloured blue directly by a dilute solution of iodine. Schacht's solution contains 1 grain of iodine and 3 grains of iodide of potassium dissolved in 1 ounce of distilled water ; but an aqueous solution of iodine answers quite well. Two substances are generally recognized in the starch grain - (1) granulose, coloured blue by iodine and forming by far the greater part of the granule, and (2) starch cellulose, not coloured blue and only forming a sort of skeleton to the grain. Starch is one of the most widely distributed substances in plants, being absent from comparatively few except the fungi.

Oil globules occur not unfrequently in the protoplasm of plants ; and in a few instances they occur in chlorophyll granules. Oil is easily distinguished by its reactions with ether, and by its optical properties.

Occasionally portions of the protoplasm assume a crystal-like ap- ( pearance, resembling cubes, octohedra, tetrahedra, &e. These porlions are known as crystalloids or protein crystals. They give the ordinary reactions of protoplasm, and differ from crystals in their power of swelling up and changing their angles in certain solutions, f as in caustic potash. Crystalloids occur frequently in the cells of the tuber of the potato, in fatty seeds, in red algx, in petals of many flowers ( Viola tricolar), and in some fruits. Usually the crystalloids occur in fatty seeds, as in the castor oil and brazil nut, in the interior of rounded grains of albunminoids, the aleurone or protein grains, along with little rounded bodies called globoids consisting of a combination of magnesia and lime with phosphoric acid. In other instances aleurone grains without crystalloids are met with, as in Cynoylossum. The aleurone grains are usually soluble in water, and are, therefore, best examined microscopically in strong glycerin, in iodine dissolved in glycerin, or in a solution of corrosive sublimate in alcohol. Aleurone grains form when the seed is nearly ripe, the crystalloids and globoids appearing earlier.

The cell-sap consists of water with different substances in solution, the substances varying in different cells, and also chancrinc, in the same cell from time to time during growth. It saturates the whole wall and protoplasm, and collects in the vacuoles and cell-sap cavity.

distinguished in the cell. When these spheres are of syrup they quickly disappear, no trace remaining in a few minutes, while, as Kraus (Bet. Zeitung, 1877, p. 329) has shown, if the substance be inulin the formation of sphierocrystals rapidly occurs, Ilesperidiu may be obtained from the unripe fruit of orange, &c., in the form of spluerocrystals when treated with alcohol.

Tannin is present in the cells of many plants, and may be seen, when water is applied to the section, in the hark of the oak or birch in the form of fine granules which soon dissolve. A. bluish black or greenish colour or precipitate is produced by the action of salt of iron, and a dark red-brown with biehromote of potash. Colouring matter (anthoeyan) gives red and blue colours to flowers and a red colour to stems and leaves, and is dissolved in the cell-sap. Lastly, calcium oxalate, which is formed in plants by the metastasis of nutrient matters (luring growth, is got rid of in many parts of plants, or rendered harmless in others, by crystallizing out, either as large crystals, prismatic or octohedral, or in masses of small crystals, or in the form of long needle-like crystals or raphides belonging to the trimetrie system. The two forms differ only in the quantity of water of crystallization present : the rollicks have two equivalents, the prisms six of water.

C ytogenesis. - The enlargement of organs of plants is not only accompanied by an increase in the size of the individual cells, bat new cells are also formed in the part, these new cells, which are at first small, soon enlarging to their full dimensions. Usually the formation of a new cell takes place by the division of the protoplasm of a pre-existing cell, the mother-cell, into two portions of equal or unequal size, the daughter-cells, These daughter-cells in turn enlarge, and may each become the mother-cells of new daughter-cells. In this way by cell-division the vegetative cells of plants are increased in number. The process of reproduction in plants is invariably associated with the formation of a new cell or cells, and in general the process is very different from that of division, there being often a diminution in the number of cells, instead of an increase.

Four types of Cytogenesis may be distinguished t - (1) llejuvenCSOCI1CC ; (2) Conjugation ; (3) Free-cell formation ; and (4) Division.

In rejuvenescence, the whole protoplasm of the mother-cell undergoes contraction and rounding ; water is eliminated, and an entire rearrangement of the molecules of the protoplasm may be noticed by changes in the contents. As a result of these changes one new daughter-cell is formed from the entire protoplasm of the mother-cell. Rejuvenescence is observed in the formation of the swarm-spores, non-sexual reproductive organs, of some algae, such as (Edogoniunt and Vaucheria, as also in the formation of single spermatozoids. The egg-cell of many algae and fungi, as well as of the vascular eryptogams, is formed by rejuvenescence, the only difference being that here the daughter-cell remains inside the wall of the mother-cell until fertilization, when it forms a wall and begins to divide.

Conjugation consists in the union of two, rarely more, masses of protoplasm, nearly or .quite similar in size and appearance, to form a single new daughter-cell, which then becomes surrounded by a wall and forms a zygospore. Time union of the two masses is always accompanied by rounding and contraction of the masses and a complete molecular rearrangement of the protoplasm. Con- jugation is seen in the group of the Conjugatm among the *re, and also in the Zyyninyreks and 2Ifyxolnycetes among the fungi. In all cases conjugation is a reproductive process. The conjugation in the 21/y.rommyeeks is very peculiar, the numerous small masses of protoplasm (the myxoaitirelae) fusing into a naked mass of protoplasm (the plasmodium).

Free-cell formation consists of the formation of several (rarely one) cells from and in the protoplasm of the mother-cell, the whole of the protoplasm not going to form daughter-cells. Free-cell formation may he typically observed in the formation of the ascospores of the Asconlyretes. The nucleus of the large mother-cell or ascus disappears, and two new ones form, which again caul again divide, thus forming eight, each nucleus forming the eentre of a new mass of protoplasm, which at length heroines surrounded by a wall. in other eases many new masses of protoplasm form after the disappearance of the nucleus of the mother-tell ; and these new masses develop a wall and a nucleus, or very rarely no nucleus forms. The endosperm in the embryo-sae of Phasealms and other phanerogams is formed by free-cell formation, the cells after attaining a certain size fusing together and forming a tissue, the individual cells of which divide. In some fungi, Peronovora, Cystupus, &c., only one daughter-cell is formed in the protoplasm of the mother-cell.

The last variety is eell-division, the whole /I' the protoplasm of the mother-cell going to form two, merely more, dough ter-vells. The process may be observed in the cells of Spirogyra, in the cells of the hairs of Tradescantia, or iu the cells near the growing points of the roots or stems of plants. Spirogyra and T# adcscantia may be observed in a hanging drop of fluid, - water in the case of Spirogyra, a dilute sugar solution (1 per cent.) in the other. In Spirogyra a ring-like groove forms round the protoplasm in the centre of the cell, gradually deepening until the nucleus divides, and the two portions of protoplasm become separate. As the separation of the protoplasm goes on, the wall forms a ring-like projection of cellulose, which gradually extends inwards until only a small central hole is left; this soon fills up, and the mother-cell is separated into the two daughter-cells.

Cell-division can he seen in the hairs of the young stamen of Tradcscantia virginica. A small unopened bud about one-fifth of an inch long is recommended by Strashurger.1 The entire stamens are removed, and one, with the small hairs attached, is to be placed in the 1 per cent. sugar solution in a hanging drop. The cover glass on the under side of which the hairs are arranged must be very thin, to permit of observation with an immersion object-glass magnifying about 600 diameters. The cell-division in the last three cells of the hair can be readily observed, as well as the peculiar behaviour of the nucleus, its solution, and the formation of the barrel-like body "Kerntonne." These and other changes, which had been fully described by Strasburger Weber Zellbildung mend Zelltheitung) in 'Spirogyra and other cells from specimens hardened and fixed in absolute alcohol, can be seen in the living cell of Trackscantia.

In the pollen of monocotyledons and the tissue cells of many dicotyledons, as in the pith and epidermis, the division of the cell differs slightly from that seen in Spirogyra. The nucleus of the mother-cell divides into two sister nuclei, and the protoplasm separates into two portions, the wall forming at once as a plate stretching right across the mother-cell and cutting it into two danghter-eells. The process of division can rarely be observed in living cells; hence it is necessary to make use of specimens killed during the process of division by immersion in absolute alcohol, or in a I per cent, solution of osmie acid.

Special modifications of the process of cell-division may be observed in yeast (Saccharoinyces), in the formation of styloconidia as in Pcnicillium, and of the basidiospores of the Basidionlycetes, as also in (Edogonium, in the sporangia of Saprolegnia, and in the spores of the higher cryptogams. In yeast a portion of the cell-wall enlarges in a sac-like manlier, and into it a portion of the protoplasm of the mother-cell passes, thus forming two daughter-eells of very different sizes ; when the smaller cell is full-grown a wall separates the two, and they become detached. In Pen/xi/Mon, and in the formation of basidiospores, a very similar process is Keit. In CErlogonium division of the cells is preceded by the formation of the curious cap-like structures at the apex of the cell due to local interealar growth of the wall Inn Saprolcgnia the protoplasm of the mother-cell divides into a large number of daughter-cells, which arc liberated as ciliated swarm-spores, and afterwards form a cell-wall. Lastly, in the spores of the higher cryptograms the division of the mother-cell into four danghter-cells is observed.

Union of Cells to form Tissues. - Cells are usually united together to form an aggregate governed by some common law of growth. Such an aggregate of cells is called a tissue. Tissues are formed in different ways, and in accordance with their mode of formation are distinguished as true and false. A-true tissue is formed by cell-division. In the young growing part of the plant the young active cells are all capable of dividing, a transverse wall cutting the mother-cell into two daughter-cells, the process being repeated for some time. In this way the tissues of the higher plant are formed, either originally from a single cell (apical cell) at the apex of the part, or from several cells (initial cells) situated at the growing point. In some of the lower plants false tissues are formed, rarely in some of the higher ones. The first mode of formation of a false tissue is noticed in some of the alga, as in Pediastrum and in Ilydrodictyon, as well as in the formation of the endosperm in the embryo-sac of many plants, as in Plectseolvs, Calcium, &c. Here the cells are at first separate and distinct, but these loose cells become aggregated together, often, as in Pediost•uni and Hydrodietyon, to form a beautiful and regular figure. In such instances the wall separating two cavities is a double structure formed by the union of two distinct walls. In the endosperm of the higher plants, when the false occurs commonly in the higher fungi, as in the mushroom.

' Cells are united. in various ways, the modes of union being often ' very characteristic of certain of the lower groups of plants, although the same modes of union repeat themselves in the higher plants. The following are the chief varieties. (1) Cell-rows have the cells united by their ends to form a long filament, formed by the repeated division of the cells. Examples of cell-rows are seen .in Spirogyra, Conferva, CElogonium, the hypliff of fungi, the month- form hairs in Tradescantia, and in many others ; not unfrequently these cells branch in various ways. Cell-fusions or vessels are cell-rows occurring in the higher plants, but having the transverse walls separating the original cells either partially or completely absorbed. They occur in the fibre-vascular bundles of plants, both in the wood and in the bast. Laticiferous vessels are examples of branching and anastomosing cell-rows. (2) Cell-surfaces have the cells united to form a single layer, and are thus in contact by the ends and sides, having an upper and under (rarely only one) free surface. Examples are aflbrded by some of the sea-weeds, as U/va, and by the leaves of liverworts. In the higher plants cell-surfaces occur not unfrequently, as in the epidermis, a layer of distinct cells, free on one surface, but in contact with other cells below. Many flat, scale-like hairs are also cell-surfaces, as well as the thin plates of cells separating the remarkable air spaces in the petioles of Naphar and Musa. (3) Cell-bundles are bands or bundles of similar cells either occurring separately or running through the other tissues of plants, and when doing so easily recognized in a transverse section of the part, as the bast-bundles in the stem of flax. Other examples occur among the red sea-weeds, and in the bundles of selerenchyma in the stems of ferns. (4) Cell-groups are small masses of similar cells, either forming the families or colonies (comobia) of many thallophytes, as Chroocoeuts, GIceocapsa, Pantiorina, &c., or forming the curious groups of selerenchyma producing the gritty particles in the pulp of the pear or the hard masses in cork. (5) Cell-masses are formed when the cells are united in all directions of space, the whole not having necessarily any definite external shape. Examples are numerous, but we may cite the tissues of large fungi, the ground tissue of the higher plants, and the pulp and hard endocarps of fleshy fruits. (6) Lastly, separate cells occur, either distinguished from the cells in the neighbourhood by their peculiar form and development (idioblasts), or the originally united cells separate themselves, as in pollen-graMs and spores, and form eremoblasts.

By the form and connexions of the cells aggregations of cells may be described as parenchymatous tissue and prosenchyY- matous tissue, both these forms occurring very commonly • in plants, and usually shortly designated by botanists parenchyma and prosenchyma. Parenchymatous cells are usually thin-walled, and have a correspondinglylarge cavity; their length is generally not very much greater than their breadth, the form frequently being rounded or polyhedral ; the walls are broad and flat, the cells, if elongated, not having pointed and overlapping ends. At the places where neighbouring cell-walls meet triangular or quadrangular intercellular spaces are formed, by splitting of the wall during rapid growth. Sometimes these spaces are very minute, in other cases they are largely developed, and if irregular growth of the wall occurs a very loose form of parenchyma may be produced, as in the pith of AlteltS. In other cases tolerably large intercellular spaces occur, as in the spongy parenchyma of the mesophyll of leaves. In prosenchyma the individual cells are greatly elongated and fibre-like, the walls arc very thick, and the cavity small or even nearly obliterated ; the ends of the cells are elongated, pointed, and overlapping those above and below; and lastly, no intercellular spaces are developed. Wood-fibres and bast-fibres are examples of prosenchyma; the young cells of stem or root, and the tissues of pith, leaves, flowers, and many fruits, of parenchyma.

By the power possessed by the cells in a tissue of divid- A ing and forming new cells such a tissue is distinguished a as meristem or the formative tissue of plants, all the other tn tissues being permanent tissues, or incapable of further enlargement by the formation of new cells. Meristem is observed forming the whole of the tissue of the young embryo plant, as also the whole tissue at the apex of a stein and root. All the other tissues of the plant are formed by the gradual differentiation of the originally similar cells of the meristem. Generally meristem tissue differentiates into special layers, each capable of forming cells which will ultimately form some definite portion of permanent tissue, not necessarily of the same value, however, in different groups of plants. The meristem of the embryo and of young stems and roots is distinguished as primary meristem, because occasionally a zone of cells forms in the permanent tissue having the characters of meristem, and secondary meristem, which either originates from the permanent tissue or is partly connected with the primary meristem. The cork-cambium or phellogen in the cortical tissues of dicotyledons is a layer of secondary meristem, while the cambium layer between the wood and bast portion of the bundle is partly (the fascicular cambium) derived from the primary meristem (the procambium) of the fibre-vascular bundle.

Walls of Tissue-Cells. - The cell-wall separating the contiguous cavities of two young cells appears as a simple homogeneous plate or lamella of pure cellulose, giving the usual reaction with Schultz's solution and iodine and sulphuric acid. As the tissue grows older and the wall thickens, it apparently separates into distinct layers having different chemical and physical properties, so that in some cases it appears as if each cavity had its own special wall separated from the neighbouring wall by a thin or thick layer of material, to which the older botanists gave the name of intercellular substance. The thickening layers usually exhibit a well-marked stratification, the strata often differing in chemical composition, as in pine-wood, in the bast of laburnum, or in the epidermis of Vinton, Ephedra, Bert' um Oleander, 3r.c. The application of Schultz's solution usually brings out the differences very well. In a few instances the middle lamellabecom es gelatinous, and swells up enormously in water. Examples are afforded by the stems of many algae, and by the endosperm of Ceratonia, where the so-called intercellular substance separates the cell-cavities widely one from the other. The middle lamella or intercellular substance and the thickening layers in the stratified cell-wall vary much in composition, but generally it is found that the incrusting layers are soluble in sulphuric acid, while the middle lamella is dissolved by nitric acid and chlorate of potash. These two substances, just mentioned under the name of Schultz's maceration process, are constantly employed to separate cells from their connexions, as the markings in the thickening layers are not injured by the solution of the middle lamella in the chlorate of potash and nitric acid.

Classification of Tissues. - ln classifying vegetable tissues it is necessary first to distinguish the different kinds of tissue depending on the characters of the individual elements composing it, and, secondly, to consider the various grouping of these kinds or species into systems more or less homogeneous and obeying certain common laws of growth. It is necessary to distinguish the kinds of tissue, because different kinds may occur in the same system, and it is further necessary to distinguish the systems, because tiro same form of cell may be repeated in different systems or in different parts of the same system and yet be of very different morphological and physiological value. In classifying the different kinds of tissue we shall fellow De Bary (Veryleiekende Anatomie der Vegetationsoryane der Phanerogamen. und Farne), and in the systems we shall adopt the threefold divisions of Sachs Lekrbuch der Botanik), now generally used.

I. KINDS OE TISSUE.

Meristem Tissue. 1. Primary. 2. Secondary.

Permanent Tissue.

Sclerenchyma.

Gland Cells.

Tracheal Tissue.

Sieve Tubes.

Laticiferous Tubes.

Intercellular Spaces.

II. SYSTEMS OF TISSUES.

Epidermal or Limitary System.

Fibro-vascular System.

Ground System.

(A.) ilferistent Tissue. - Primary meristem can bo observed in the embryo in its young stages, and at the apex of the stem and root. In the embryo at an early stage, as described by Hanstein (Botanische Abhandlungen, i.), the meristem becomes separable into three zones, differing in the appearance and arrangement of the cells and in the mode of dividing. These zones were called by Hanstein (1) dermatogen, or primary epidermis ; (2) the plerome or central series of more elongated cells with marked longitudinal division of the cells; and (3) a series between the plerome and dermatogen, dividing more or less irregularly or transversely, the periblein. These three zones remain distinctly marked at the apex of the stem, and in Hippuris the three can be easily seen, while as the stein elongates new cells continually form, the initial cells or cell, as there may be one or more for each zone. Usually the dermatogen layer is the most constant in angiospermous plants, the separation into periblem and plerome being sometimes a little obscure. In the root a fourth zone of meristem has to be distinguished, called by Janczewski the calyptrogen layer, from which the calyptra, pileorbiza, or root-cap is formed. Various modifications of the arrangement of the different layers in the meristem of roots have been described. Very rarely, as in Ilydrocharis and in Pisan, four distinct layers are formed - the plerome, periblem, dermatogen, and calyptrogen. In Cucurbita, Pisum, and a few others there is a common mass of meristem at the apex, from which the others are all differentiated. In Zea Mays and most monocotyledons two distinct zones aro seen, the plerome and the calyptrogen, while between theni a short distance from the apex the initial layer forms, which separates into the periblem and dermatogen. In Fagopyrum and most dicotyledons the plerome and periblem are sharply separated, but the periblem above the apex of the plerome passes into a common layer with the initial cells of the dermatogen and calyptra, the dermocalyptrogen. In gymnosperms the root possesses a sharply-defined plerome with a periblem mantle, in Thuja formed by from 12 to 14 regular concentric layers ; there is no trace either of a calyptrogen or derrnatogen layer, the outer cells of the periblem serving as a calyptra. In the stems of gymnosperms the condition of the layers is somewhat intermediate between those formed in the angiosperms and lycopods. In A raiccaria and Dammara the dermatogen, periblem, and plerome are separate and distinct, but in AlrietineT, and in Cycas they run into a common initial group, and it is only at some distance from the apex that in the Abietineoe the separation becomes very marked, and in Cycas only slightly marked. In lycopods the end of the stein shows a series of cells, the initial group from which the periblem and dermatogen (or the external layer representing it) arise. Further down the initial cells of the plerome are developed from the side of the periblem. In the root of lycopods the arrangement of the layers is exactly the same as in Ilydrocharis and Pistia. In the Ligulatw and the remaining Pteridophyta there is a single cell at the apex of root and stem which divides into two. The one daughter-cell forms the new apical cell, the other is the segment cell. The segment cell divides still further, and forms a meristem from which at a later stage zones corresponding more or less accurately to dermatogen, periblem, and plerome are produced. In the roots a segment is cut off in front of the apical cell, which is the first cell of the calyptra, and from which, by repeated divisions, that structure arises.

Secondary meristem is intimately connected with the secondary circumferential growth of stems and roots in gymnosperms and dicotyledons. One of the zones of ' secondary meristem arising from permanent cells is the cork-cambium or phellogen layer, which is described under the epidermal system of tissues. The other example of secondary meristem is the cambium layer separating the wood and bast in the stems and roots of gymnosperms and dicotyledons.

When the fibre-vascular bundles first appear, either in ] the periblem or plerome, the cells become distinguishable by I their form and arrangement, and as the cells are still in the condition of meristem, the term procambium has been given to the whole. The cells of the procambium are gradually converted into permanent tissue, generally changing their appearance completely, although in some cases the change is but slight, the cells being cambiform and hardly differentiated into the two parts of the fibre-vascular bundle, the wood and bast, to be described under the fibre-vascular tissues. In some plants all the procambium is converted into permanent tissue, while in others a small zone between the wood and bast remains in the condition of meristem. If the bundles are separate, secondary meristem forms in the ground tissue between the bundles, bridging over the space between the bundles, but uniting so as to form the cambium-ring, which consists of fascicular cambium in the bundle, derived from the procambium, and interfascicular cambium, a secondary meristem formed in the ground tissue. It is by the growth of this cambium ring that the secondary circumferential growth, so marked in our ordinary forest trees, takes place.

(B.) Permanent Tissue. - It will be sufficient to give only a general sketch of the seven kinds of tissue described by De Bary, and to refer for full details to his Veryleichendo Anatomic above mentioned.

Cell-tissue is permanent tissue, the cells of which are little if at all altered in form and appearance from their meristem stage. In some cases the cells are short, in others elongated. The wall may be thin, and enclose the protoplasm and othercontents, the chlorophyll, starch, sugar, inulin, &c. In others the wall is thick and changed in composition. As varieties of cell-tissue lle Bary includes (1) epidermis and its appendages, equivalent to the epidermal system of Sachs, and to be considered below; (2) cork, parenchymatous cells chemically altered, and forming usually a part of the secondary epidermal system ; and (3) parenchyma proper, all the cell-tissue inside the epidermis and cork cells, a division almost but not quite equivalent to the ground tissue of Sachs.

Seletenehyma. - De Bary includes under the name of selerenchyrna all the hard thickened cells of plants, whether long or short, i which have become greatly thickened, and whose cavity is nearly if not quite obliterated, - the cell-contents also, as a consequence, having entirely disappeared, or left only slight traces. In this state these cells act in conveying water through their walls, and also serve to give rigidity to the plant, forming the mechanical system of Schwendener. Two forms are distinguished: (1) the short sclerenchymatous cells, and (2) long sclercnehymatous fibres. Of the former, examples are nia with in the flesh of the pear, in the root-tubers of Dahlia, in the rhizome of Dentaria, the pith of Hoya carnosa, and many others. Such cells are rare in monocotyledons, and the typical form (like the cells in the pear) does not occur in cryptogams. A variety of the short cells is described under the name of stegmata. The long sclerenchymatous fibres are pointed, with overlapping ends, and occur commonly in dicotyledons. They are either simple or branched. The best examples are the bast fibres of the fibro-vaseuIan bundles, and the libriform fibres of the secondary wood. The wall of the sclerenchym fibre often exhibits peculiar split-like pitted markings (Ptcris aquilima). Not unfrequently the sclerenehym fibres have numerous small crystals of calcic oxalate imbedded in the wall, a very beautiful example of which is afforded by 1Velwitschia mirabilis. Sometimes the cavity in the interior of the fibre is divided by transverse partitions forming chambered fibres, as seen in the bast of the vine, Platanus, Taman.; Tracheal Tissue. - Under this head De Bury distinguishes all those cells which become more or less lignified, and in which the thickening of the wall assumes the form of spirals, rings, reticulations, or pits, and which as soon as these markings are formed either lose their contents completely and become filled with air, or contain clear watery fluid. Usually these form long cell-fusions, the vessels of plants, or else they form elongated or shorter cells not united into a vessel. The former are the vessels, the latter the tracheides. The markings in the two forms correspond, and there are intermediate varieties. The markings are spiral, annular, reticulated, pitted, and trabeculate (juniper and lycopod), with the varieties of bordered pits and scalariform markings. Short tracheides form the velamen or outer modification of the epidermis of the aerial orchid roots, also the outer tissue of the stein of Sphagnum. In Nelumbriumspeciosum the tracheides are 12 centimetres long. Many of the structures usually called vessels are tracheides. Large vessels frequently exhibit tyloses or cells filling up the cavity of the vessel. They have been observed in many monocotyledons and dicotyledons, both in stems and roots, and in herbaceous as well as in woody plants.

Sieve-tubes. - These resemble vessels in being elongated cylindrical or prismatic cells joined in long rows, the individual cells always remaining distinctly marked. The transverse wall separating the two cavities becomes perforated at the unthickened parts, forming the sieve-plate perforated by the sieve-pores. The contents of the sieve-tubes are colourless and transparent, and the wall is coated with a thin layer of protoplasm-like substance, not unf•equently with small starch granules. Sieve-tubes form a special part of the bast of plants, and are met with in pteridophytes, gymnosperms, and angiosperms, exhibiting occasionally in different groups slight structural differLaticifcrous Tubes are tubes containing the peculiar milky sap or latex occurring in special groups of plants. These run through the plant usually for very long distances, and when a portion is injured the milk-sap flows out at the opening. The walls are always soft, of pure cellulose, and readily giving the characteristic reaction with iodine and sulphuric acid. The tubes contain no protoplasm and nucleus ; but a quantity of a rarely watery, usually milky juice, occasionally, however, orange or yellow, and sometimes containing peculiar starch granules. The tubes are either simple or segmented. Segmented tubes occur in Ciehoracece, Campanulacew, Lacliacca Papayaccce, many Papaveracccc, as Papaver, Argemone, and Chc/idonium, but not in Criaucium or ,S'antruinaria, many aroids, and Musaceec. Simple tubes are met with in E'uphorbiaccec, Urticaccx, A poeynacecc, and Aselepiad(Areca. These latter do not exhibit the net-like anastomoses of the segmented forms, and usually have the branches terminating in blind extremities.

Intercellular spaces are the cavities between the elements of full-grown tissues, the cells in the meristern stage being in unin terrupted continuity. Some of the intercellular spaces are produced ( by the splitting of the cell-wall between three or more cells, others i are formed by the destruction of the walls of a cell or group of cells t during the formation, by desorganization, of some secretion. Lastly, large cavities appear in plants as the result of mechanical rupturing and tearing of the inner tissues during rapid growth of the part. De Bary distinguishes all these by separate names, viz., schizogenous when formed by splitting of the common wall between cells, lysigenous when formed by the destruction of certain cells and cell-groups, and rhexigenons when produced by mechanical disruption. From the nature of the contents, the intercellular spaces can be divided into two groups, the one containing substances or mixtures similar to those contained in gland-cells, the other containing air, or rarely water. Of the intercellular glands, spaces, or canals the following varieties may be distinguished: - (a.) mucilage or gran canals, of which examples may be seen in .3farattlacecc, Lycopoclicteecc, Cycadacece, Canna, Opuntia, and some Araliacece ; (b.) resin, ethereal oil, or gum-resin canals, either in long canals, as in Conifercc, Alismacece, aroids, Compositce (Tubuliflorce), Umbelliferce, Araliccecce, &c., or short spaces as in Rutacece, Hypericum, Oxalis, IlIyrtacecc, Lysimachia, &c. Of the intercellular air or water spaces there are several modifications. First there are the minute spaces between the walls of parenchymatous cells, the interstitial air spaces ; and when the spaces are larger and accompanied. with irregular growth of the wall, lacunae are produced, as in the root of Sagittaria sagittifolia, or in the pith of Juncus or petioles of Musa, he. Large schizogenous air-spaces with smooth walls are met with in Isoetes, Potamogclon, Ilippuris, Trapa, Nymphceaccce, and many others. Lysigenous spaces having the remains of the destroyed cells more or less marked on the walls are seen in Equisetum, Cyperaeow, Graminece, Typlat, Iris ; while the large hollow stems of Umbelliferw, Composike, grasses, and the leaves of Alli>em, he., are rhexigenons. Occasionally flat cell-surfaces or diaphragms interrupt the continuity of long air-spaces, and not unfrequently internal hairs or peculiar hair-like idioblasts are formed, projecting into the intercellular spaces as in ,(Vnphar and Monstera. It is only in the neighbourhood of water stomata that the spaces contain water for a short time.

Systems of Tissues. - Sachs describes three systems of tissues, complex aggregations consisting of different kinds of tissue, but all so combined as to form readily recognizable parts of the root, stem, or leaf of a plant. Externally there is the epidermal or limitary system equivalent to Dc Bury's first division, excluding his parenchyma. This system is taken to include the epidermis of plants, with its cuticle, stomata, and hairs, and also to include the secondary modifications produced by the development of cork and bark. In the interior of most parts of the higher plants, and following in the direction of the long axis of growth, separate or united strings or bundles are seen running and usually branching or anastomosing. Generally these bundles are harder than the surrounding tissues and readily separable from them. Consisting as they do of many kinds of tissue of vessels, cells, and sclerenchyma, these structures are known as fibre-vascular bundles. Lastly, there exists a quantity of parenchyma or a mixture of parenchyma with other forms, packing up all the space between the fibrovascular bundles on the one hand and the epidermal system on the other. This forms the ground tissue, and includes the parenchyma proper of De Bary.

The epidermal system takes its name from the chief member of ] the group, namely, the epidermis or outer skin of the plant. It is the superficial layer, and is variously developed in the higher and lower plants. In the lower forms, alga, fungi, lichens, the external cells are usually smaller than those below-, or the walls are thicker and coloured ; while in many mosses and liverworts a true separable epidermis is only slightly indicated. In others, as in Marchaidia, capsules of most mosses, and in Sphagnum, a specially differentiated epidermis appears, resembling that in the higher plants. The nature of the epidermis varies in accordance with the conditions to which it is exposed, as to air and light, or in water, or in the soil, and in darkness. The nature of the limitary tissue also varies with the stage of growth in such pails as are of perennial duration.

Usually the epidermis is a single layer of cells producing stomata and hairs. In many plants the epidermis is strengthened by the formation of a corky outer layer, the cuticle, which develops wax ; or in other cases a new formation takes place below the epidermis, usually in the ground tissue, and by the formation of layers of cork a secondary epidermal or limitary tissue is produced. Other parts of the ground tissue assist in forming the outer covering of plants, and may be considered physiologically to belong to the epidermal system. These will be described as hypoderma and collenchyma under the ground tissue.

Epidermis. - The cells of the single layer forming the epidermis vary in shape, but usually the form is determined by the shape of the part on which they are developed, being elongated on long leaves, broad with straight or wavy margins on broad leaves. Usually the cells of the epidermis, although parenchymatous, have no intercellular spaces, except in Osinuncia and Todca, and a few other rare instances. The onlyopenings are those in the stomata, schizogenous intercellular spaces, between the special cells (guard cells) of the stoma. In many plants, as monocotyledons and needle-leaved conifers, the epidermal cells contain no chlorophyll ; but in ferns and iu many dicotyledons, as has been shown by Stoehr (Bet. Zeit., 1879, p. 581), chlorophyll is present. Not unfrequently authocyan fills the epidermal cells, and completely obscures the green colour of the chlorophyll-bearing cells below. The outer wall of the epidermal cell is usually greatly thickened and corky, forming the cuticle, which generally forms a continuous sheet separable by the action of caustic potash from the rest of the wall below. In applying Schultz's solution to a thin section of an epidermal cell, the outer layers become brown, while the inner give the reaction of cellulose. The outer layers are soluble in boiling caustic potash and in nitric acid and chlorate of potash, but insoluble in sulphuric acid and in ammoniacal solution of cupric oxide. Many of the cells have a marked deposit of mineral matter, more particularly silica (Equisetam), in their walls. See Nageli and Schwendener, Das Mikrosk.op (2d ed.), p. 489.

Wax is frequently produced : either it is on the surface of the cuticle forming a variously constructed coating, or minute particles are embedded in its texture. The chief modifications are described by De Bary ( Vergleieherlde Anatomic, p. 86) : - (1) a layer or crust, either thin, homogeneous, and transparent, or thick and striated, the former seen in Sempervivion, the latter in the waxpalm (K/opstockia); (2) a coating of rod-like particles placed perpendicularly to the surface, either closely placed or somewhat loose and irregular(Saceharuin, Mesa, and Seitaminex); (3) a layer of granular particles, close or widely separated, and not placed one over the other (Allium, Acer, Vitis), &c. ; and (4) irregular granules piled up one over the other in several layers, as in Eucalyptus, Rid7122S, Abies pectinata, &c. t, Stomata (De Bary, Vergleich. Anat., p. 36 sq.) are the openings in the epidermis which permit the entrance and escape of gases. They are formed by two semilunar cells, the guard cells, with the pore or intercellular space between them, the pore opening into a large air-space in the tissue below, and in communication, by means of the small intercellular spaces of the parenchyma, with most of the tissues of the plant. The stomata are found on those parts above ground exposed to air and light, hence chiefly on the leaves and tender green stems of plants. On leaves they are most abundant on the under side, and are generally absent from the upper surface. In many leaves, however, especially of monocotyledons, they are equally distributed on both sides, and in water-plants with floating leaves they are abundant on the upper side but absent from the lower, They rarely occur on submerged water-plants and never on roots. As a rule the stomata are irregularly scattered, but in some plants, as in Equisctum, they occur in tolerably regular longitudinal rows on the stem. Usually the stomata consist of only two cells, the guard cells, or of two pairs of guard cells (Equisclum) one over the other, or there are many, as in the peculiar stomata of Merchant in. In some plants two or more additional cells, the accessory cells, are formed. These accessory cells differ from those of the epidermis on the one hand, and from the guard cells on the other. The position of the stoma varies. It is sometimes at the end of the long epidermal cells, as in the hyacinth, or at the side, in a few cases free in the centre of the epidermal cells (Anemia, Sze.). The guard cells may be on a level with the epidermis ; rarely they project slightly ; but frequently they are depressed below the surface. The guard cells often contain chlorophyll and starch, the outer wall is often thickened, and occasionally even wax forms on their surface ; but as a general rule no wax forms, and thus, when a thick coat of wax is developed, narrow canals through it indicate the position of the stomata.

Der elopnzent of Stomata. - In long epidermal cells (hyacinth) a portion is cut off at one end by cell-division, and forms the mother-cell of the stoma. It then divides into two daughter-cells, each forming one of the guard cells. The lamella between the two splits, either from without inwards or within outwards, and forms a schizogenons intercellular space. When the epidermal cells are not elongated (CEnothera, Slime, &c.), a portion of the epidermal cell is cut off at one part by a bent wall. This is the mother-cell of the stoma, and either forms the daughter-cells immediately, or may divide by segments cut off at one side and then at the other side, either one, two, or more times before the central cell divides to form the daughter-cells which form the guard-cells of the stoma. The other cells cut off on each side are the accessory cells. In other cases the accessory cells have a different origin, being cut off from the neighbouring epidermal cells alter the guard-cells are formed. Instances of the former may be seen in Crassulaeece, Cruciferec, and Puptilioneteece ; of the latter in Juncaceee, Cyperaecx, and. Gramincce.

In Anemia and some other ferns a cell is formed inside the epidermal cell, cutting a cylindrical piece out of it. This divides and forms the guard-cells of the stoma.

Two kinds of stomata exist in many plants. The one kind, already fully described, are the air-stomata, to distinguish them from the s second kind, the water-stomata. The latter occur in many plants on the leaves, immediately over the ends of the fibro-vascular bundles, near the margin on the upper surface, and often on the serrations of the margin itself. They give off water, during a portion of the life of the leaf, which appears on the surface in the form or drops, under the action of root-pressure. They are at once distinguished by their large size, and by their not opening and closing like air-stomata.

Hairs (Dc Bury, Vcrgleieh. Anat., p. 58) are usually ;ut-growths of single epidermal cells ; but occasionally some of the cells below the epidermis assist in the construction of large massive hairs or emergences, as they are called by Sachs. Hairs vary very much in construction, size, and appearance, and not unfreqnently different kinds of hairs occur mixed together on the same part of the plant, although in many instances only a single characteristic variety of hair may be developed on the epidermis. De Bary distinguishes several typical varieties of hairs : - (1) hairs proper ; (2) papuloe, short rounded sac-like structures ; (3) scales ; (4) villi ; and (5) warts or prickles. The simplest hairs are outgrowths of single epidermal cells, having the cavity either continuous with that of the epidermal cell, or cut off by a wall. Long cylindrical unicellular hairs occur in cotton ; and on most roots root-hairs, with thin or sometimes with peculiarly and irregularly thickened walls (Viola tricolor). The cells may divide and form a moniliform hair, as in Traclescantia, or much more complex branched ( Vcrbascum thapsus) or club-shaped and glandular hairs may be produced. Flat, dry scales, either unicellular or multicellular, are seen iu Deutzia, Eltragnus, and in many ferns. Papule are mentioned by Dc Bary as occurring on Roches, Begonia, Piper, Ampelopsis, and others. The villi or colleteres occur on bud-scales and buds, while spiny hairs or warts occur frequently as the prickles of the rose and bramble, and in Dipsacns, Smilax, &c. The walls of hairs are often thin, and composed of nearly prim cellulose, or thickened and stratified in various ways, with an outer cuticular layer. The thickening is either general or local, and may assume the form of pores or spiral striation (hairs on stamen of Bulbine abides), or may form peculiar warts or nodules. Silicions hairs (Dentzia), or hairs containing lime, sometimes occur. In some cases the hairs (nettle) are supported on cellular, elevations of the epidermis. These may be distinguished as the accessory cells of the hair. Glandular hairs are of frequent occurrence, the end cell or cells secreting some ethereal oil or resin ; the secretion collects below the cuticle, and either it remains there, causing the absorption of the secreting cells, or the cuticle ruptures. The villi or colleteres are peculiar many-celled glandular hairs on young leaves, stipules, or bud-scales (Ribes, Viola, Polygonum, ./Eseulus), and secreting a guns or resin. Frequently the secretion of these colleteres is supplemented by the formation of a resin from below the cuticle of the epidermis, forming the gelatinous secretion covering buds, termed blastocolla (horse•chestnut). In some plants, as Populus, the blastocolla is formed by the epidermal cells alone, in others both by the colleteres and epidermis.

Beneath the epidermis the cells are often peculiarly modified to form the hypoderma and collenchyma ; but as these belong to the t ground system of tissues they are described below. The secondary r epidermal tissues, or the covering that replaces the epidermis on the t perennial parts, consists largely of cork, either in the form of a thin I layer or in repeated layers developing deeper and deeper in the tissues o of the stem or root and forming the massive bark or rhytidome. Cork r cells arise usually from the cortical cells, i.e., those of the ground tissue placed a short distance below the epidermis (Populus, Sambucus). In other cases the cork forms still deeper, among the green chlorophyll-bearing cells of the cortex, as in _Rubus Idwus, Ice. Rarely the cork-cells arise from the epidermis itself (Salix). In all cases cork is formed by the division of the cells of the cortex or epidermis by a tangential wall, separating the mothercell into two daughter-cells. The outer cell becomes corky, rapidly losing its contents and becoming filled with air ; while the inner one retains its protoplasm and forms new cork-cells by division. The formation of cork does not necessarily begin at all parts of the circumference simultaneously, hut sooner or later a complete layer of cork is formed. When the layer has become a few cells thick, it is known as the periderm ; while the active cells from which it arises arc distinguished as the cork cambium or phellogen. lusidr the cork cambium new cells are often formed, which contain chlorophyll, and are known as the phelloderma (Fat/us, ,$'<r/ix), such cells being also funned by division of the cork cambium. A fter the formation of the periderm, as is easily seen in the stem of the black currant, the whole of the epidermis and of the ground tissue immediately below becomes withered, and is thrown off. In the formation of bark, the. layers of cork form repeatedly in the cortical tissue of the stem, and even in the bast portion of the libro-vascular bundles. The layers of cells between the plates of cork, being cut off from a supply of nourishment, soon wither ; and thus occasionally the dead parts scale off, as in the Platanus, cherry, &c. The bails or rhytidome is thus a very complex structure, consisting of the secondary epidermal tissues either formed in the primary- cortex alone or deep in the other tissues, and popularly it includes all the tissues outside the cambium layer, that is, the bast part of the fibro-vascular . bundles and secondary epidermal tissues. Lenticels are special structures connected with the epidermal tissues, and are common on dicotyledons (Sambucus, Populus,Juglans, &e. ), and on some monocotyledons, being formed on stems, branches, petioles, and roots. Below a stoma or group of stomata a few cells enlarge and divide, and form numerous colourless thin walled cells, which arise from the bent layer of lenticel cambium below. The epidermis becomes ruptured and the cells appear on the surface, forming a brownish wart-like marking. These lenticels are probably to be considered functionally as secondary stomata, as the cells have large intercellular spaces and readily permit the passage of air into the interior. Lenticels have the marked peculiarity of being sometimes closed in autumn by the formation of cork cells, but open again in spring.

String-like bundles, the fibre-vascular bundles, are common in vascular cryptogams, gymnosperms, and angiosperms, and are familiar in the leaves of plants as the veins. They run in the ground tissue either separately or united, as in many dicotyledons, and in most roots, &c., to form a central or hollow cylindrical vascular mass. When the bundles are separate they often branch and anastomose as in leaves, or they may only auastomose at the nodes of stems. The bundles are easily separable by maceration, except in water plants, and a few others, in which the bundles are very soft ; or they may be examined in transverse and longitudinal sections of the part, more particularly in the latter case when the tissues have been rendered transparent by boiling in dilute caustic potash, or by being previously boiled in strong nitric acid. (See Niigeli and Schwendener, Das Mikroskop, p. 632.) Each bundle in its perfect state consists of two groups of cells, the wood or xylem portion, and the bast or phloem. Bundles are either closed or open. In the former the procambium cells, the . meristem, from which the permanent tissue of the bundle originates, entirely passes over into permanent tissue ; while in the latter the cambium remains between the xylem and phloem, and is capable of forming new cells for an indefinite period. Closed bundles thus rapidly assume a permanent form, while open bundles go ou growing. Fibro-vascular bundles are divided by De Bary into four groups by the mode of arrangement of the xylem and phloem. The first and commonest form is tho "collateral" bundle, where the xylem and phloem are placed side by side with or without cambium between them, the xylem being always towards the pith or the central part of the stem, the phloem external. In Cueurbita, Solanum, and others the bundles are "bicollateral," there being an additional phloem portion inside the xylem. "Concentric" bundles occur in many vascular cryptogams, the central xylem being completely sur- rounded by the phloem. The last form is the " radial," where the bundles of phloem and xylem are arranged alternately in the central fibro-vascular axis, as in most roots. Irregular bundles also occur, and numerous intermediate forms connect the different types.

In each of the portions of the bundle different kinds of tissue occur ; but there is a marked similarity in the construction of the phloem . and xylem, at least in separate bundles and before circumferential growth takes place. In the wood, distinguished by the dignified hard brittle walls of the cells, there are four elements usually present: the wood vessels or cell fusions filled with air, having the transverse walls more or less completely absorbed, and having thickened walls marked with rings, spirals, reticulations, or pits of different kinds ; the ends of the cells sometimes are more or less pointed and overlapping, with pitted markings, having, however, a free communication front cell to cell through the absorbed thin part of the pits; tracheides, or vessel-like wood prosenchymatous cells, having walls marked like the vessels, and with the cavity containing air, but never showing any absorption of the end walls and fusion into vessels ; (3) wood prosenchyma or libriform fibres, elongated, pointed, and overlapping cells, exactly resembling bast fibres, often with greatly thickened walls, these walls never having spiral or annular markings, but only small simple or occasionally exceedingly minute bordered pits ; they are very common in the wood of dicotyledons, and may either be simple or have fine transverse partitions forming chambers in the long cell ; (4) wood parenchyma, wood cells with thin walls, and simple pits ; these in winter contain starch, and other reserve materials, along with the cells of the medullary rays, and at other times may contain tannin, chlorophyll, a or crystals of calcium oxalate. in the lust or phloem portion of the bundle there are three elements only, as there are mm cells equivalent to the tracheides. These are - (l) the sieve tubes or bast-vessels, cell-fusions like the wood vessels, but having the transverse portion forming the remarkable sieve plate perforated by the sieve pores, while occasionally such plates or similar markings our on the side walls: the walls are soft and delicate, giving a cellulose reaction, and the cavity contains abundant protoplasmic contents with excessively minute starch granules ; (2) bast prosenchyma or bast vessels, elongated prosenchymatous cells, with pointed and overlapping ends, the walls so thick as almost to obliterate the cavity ; the walls are soft and flexible, often marked with fine pits; like the libriform fibres of the wood, they have occasionally the cavity chambered with thin transverse walls, and not unfrequently they branch ; (3) bast parenchyma, repeating the wood parenehyma ; but occasionally the cells are long and narrow, exactly like those of only slightly modified procambium, which they really are ; in this state they are often called cambiform cells. The sieve-tubes and bast parenchyma or cambiform cells form the soft bast. These different elements of the wood and bast are not always present, and the secondary wood and bast developed from cambium are often very different from the primary portions developed front ibrocamhimn. Thus in Cueurbita there are no Last fibres, while in most coniferous woods the tracheides alone are present in the xylem. At the ends of the fibro-vaseular bundles in the leaves the different elrments gradually disappear until one or two spiral vessels awl a few cambiform cells alone remain. In most roots the fibre-vascular bundles form a central mass with the phloem and xylem in separate groups and arranged alternately ; the xylem masses generally project into the centre, and the oldest vessels are nearest the centre. The whole mass, which is either a single bundle or a group of bundles, is usually surrounded externally by a peculiar layer, the perieambimn, in contact with the endodermis or sheath, the inner layer of the ground tissue, which in roots forms the massive cortical portion.

The ground tissue comprises all that remains after the formation of the epidermal and fibro-vascular systems; and is usually composed ofparenchymatous cells, not in any way distinguishable except by their position from parenchymatous cells in the other systems. In other cases the ground tissue contains prosenchyma, or the cells in certain regions are more or less thickened. When the part contains closed fibro-vascular bundles, as in monocotyledonous stems and in leaves, the ground tissue forms the chief bulk of the part ; but in other eases, as, for instance, in the stems of conifers and dicotyledons, with circumferential growth, the ground tissue is very feebly developed. In such stems the ground tissue forms the pith and cortex, with the primary medullary rays joining the two. In roots with a central fibro-vascular mass, the cortex is the only part of the ground tissue represented. The ground tissue immediately below the epidermis may be simply parenchymatous, or it many exhibit certain modifications. Either the cells form collenchytna, as in many stems and petioles, a tissue consisting of mere elongated cells without intercellular spaces, and having special masses of thickening matter developed on the walls where neighbouring cells meet. These masses readily swell up in water, and probably act as a sort of ereetile tissue. In other cases a greater or less development of hypoderm a is observed in leaves and stems, the cells being elongated and greatly thickened and sclerenchymatons, resembling in most points the bast-fibres of thefibro-vaseular bundles. In some plants, as in ferns, separate, often dark-coloured, bundles of sclerenchyma occur in the ground tissue. These different elements form part of what has been distinguished as the "mechanical" system of tissues, hardened cells giving rigidity to the different parts of the plant, and although such cells occur in very different parts of plants, as in fibro-vascular bundles and in the ground tissue, still they have a marked external resemblance, and are closely related physiologically. Thick, short, sclerenchymatous cells occur in the ground tissue, as in the pulp of the pear ; in other eases the parenchyma is unthickened, and contains either colourless contents or develops chlorophyll. The part of the ground tissue next the fibro-vascular bundles forms the sheath or endodermis, a layer of cells often thickened or cutieularized, and surrounding either single bundles or the whole vascular mass or series of fibrovascular bundles. In some cryptogams the endodermis is strengthened by numerous sclerenchymatous cells surrounding it either partially or completely. The ground tissue of tree Liliaccas, and even in some abnormal dicotyledons, forms a layer of secondary meristem cells capable of developing both new ground tissue and new fibro-vaseular bundles ; and it is in this way that the secondary circumferential growth in the stems and roots of Draceena, and probably of the fossil vascular cryptogams, took place. The secondary circumferential growth of gymnosperms and dicotyledons is the result of the activity of the cambium ring formed by the fascicular cambium and the interfascicular cambium in the ground tissue, as already described. The changes produced by secondary circumferential growth are very numerous, and are fully described by Do Bary ( Vergleiclumle Anatomic, chaps. xiv. and xv.).