REPRODUCTION OF PLANTS - The various modes by which plants are reproduced may be conveniently classified in two groups, namely, vege-tative multiplication and true reproduction, the distinction between them being this, that, whereas in the former the production of the new individual may be effected by organs of the moSt various kinds, in the latter it is always effected by means of a specialized reproductive cell. This distinction will become apparent in the following discus-sion.
Vegetative _Multiplication. - The simplest case of vege-tative multiplication is afforded by unicellular plants. When the cell Nvhich constitutes the body of the plant has attained its limit of size, it gives rise to two, either by division or gemmation ; the two cells then grow, and at the same time become separated from each other, so that eventually two new distinct individuals are produced, each of which precisely resembles the original organism. A good example of this is to be found in the gemmation of the Yeast plant. This mode of multiplication is simply the result of the ordinary processes of growth. All plant-cells thus grow and divide at some time of their life; but, whereas in a multicellular plant the products of division remain coherent, and add to the number of the cells of which the plant consists, in a unicellular plant they separate and constitute new individuals. In more highly organized plants vegetative multiplication may be effected by the separation of the different parts of the body from each other, each such part subsequently developing the missing members and thus constituting a new individual. This takes place spontaneously, and in a marked manner in Mosses. The main stem gradually dies away from be-hind forwards ; the lateral branches thus become isolated, and constitute new individuals.
The remarkable regenerative capacity of plant-members is largely made use of for the artificial propagation of plants. A branch removed from a parent-plant will, under appropriate conditions, develop roots, and so con-stitute a new plant ; this is the theory of propagation by " cuttings." A portion of a root will similarly develop one or more shoots, and thus give rise to a new plant. An isolated leaf will, in many cases, produce a shoot and a root, in a word, a new plant ; it is in this way that Begonias, for instance, are propagated. The production of new plants from the leaves occurs also in nature, as, for instance, in the so-called " viviparous " plants, of which Bryophyllum calycinum (Crassulaceoe) and many Ferns (NePhrodium [Lastrvea] Filix-nzas, Asplenium [Athyrium] Filix-frizina, and other species of Asplenium) are examples. An interesting case of the vegetative develop-ment of new individuals from other plant-organs is afforded by Strasburger's observations on Ccelebogyne ilicifolia, Funkia, Nothoscordunt fragrans, and Citrus ; he found, namely, that in these plants, an adventitious formation of r embryos takes place by budding from the tissue of the nucellus bounding the embryo-sac. But it is in the Mosses, of all plants, that the capacity for vegetative propagation is most widely diffused. Any part of a moss, Nvhether it be the stem, the leaves, the rhizoids, or the sporogonium, is capable, under appropriate conditions, of giving rise to filamentous protonema on which new moss-plants are then developed as lateral buds.
In a large number of plants we find that provision is made for vegetative multiplication by the development of f more or less highly specialized orga,ns. In Lichens, for instance, there are the soredia, Nvhich are minute buds of the thallus containing both algal and fungal elements ; these are set free on the surface in large numbers, and each grows into a thallus. In the Characem there aro the bulbils or " starch-stars " of Chara stelligera, which are B underground nodes, and the branches with naked bctse and the pro-embryonic branches found by Pringsheim on old nodes of Chara fragilis. In the Mosses small tuberous bulbils frequently occur on the rhizoids, and in many instances (Bryunz annotinum, Aulacomnion anclrogynunz, Tetraphis pellucida, Stc.) stalked fusiform or lenticular multicellular bodies containing. chlorophyll, termed gemmx, are produced on the shoots, either in the axils of the leaves or in special receptacles at the summit of the stem. Gemmm of this kind are produced in vast numbers ia Marchantia and Lunularia among the Liverworts_ Similar gemm are also produced by the prothallia of Ferns. In some Ferns (e.g., Nephrolepis tuberosa and N. undulata) the buds borne on the leaves or in their axils become swollen and filled with nutritive materials, con-stituting bulbils which fall off and give rise to new plants. This conversion of buds into bulbils which subserve vegetative multiplication occurs also occasionally among Phanerogams, as, for instance, in Lilium bulbiferum. But many other adaptations of the same kind occur among Phanerogams, notably among annuals. Bulbous plants, for instance, produce each year at least one bulb or corm c from which a new plant is produced in the succeeding t' year. In other cases, as in the case of the Potato, tubers are developed from subterranean shoots, each of which in the folloNving year gives rise to a new individual. In other cases, again, a.s in Dahlia, Thladiantha dubia, tuberous swellings are found on the roots, from each of which a new individual may spring.
True Reproduction. - It was mentioned above that the .9 true reproduction of plants is effected by specialized cells ; c these cells may be generally designated spores. The structure of a spore is essentially this : it consists of a nucleated mass of protoplasm, enclosing starch or oil as reserve nutritive material, usually enclosed by a cell-wall. In those cases in which the spore is capable of germinating immediately on its development the cell-wall is a single delicate membrane consisting of cellulose ; but in those cases in Nvhich the spore may or must pass through a period of quiescence before germination the wall becomes thickened and may consist of two layers, an inner, the endospore, which is delicate and consists of cellulose, and an outer, the exospore, which is thick and rigid, frequently darkly coloured and beset externally with spines or bosses, and which consists of cutin. In some few cases among the Fungi multicellular spores are produced ; these approximate somewhat to the genam mentioned above as highly specialized organs for vegetative multiplication. In some cases, particularly among the Algm and also in some Fungi (Peronosporew, Saprolegniere, Myxomycetes, Chytridiacex), spores are produced which are for a time destitute of any cell-wall, and are further peculiar in that they are motile, and are therefore termed zoospores ; they move, sometime,s in an ainceboid manner by the protrusion of pseudopodia, but more frequently they are provided with one, two, or many, delicate vibratile protoplasmic filaments, termed cilia, by the lashing of which the spore is propelled through the water. The zoospore eventually comes to rest, withdraws its cilia, surrounds itself with a cell-wall, and then germinates.
Spores are developed in various ways, and a prefix is in many cases added to the word " spore " to indicate the nature of the process of development. Leaving details for subsequent discussion, we will now confine our attention 1 to the main fact that all spores are developed in one or other of two ways, either asexually or sexually. In the former case a single reproductive organ gives rise to cells which are capable, each by itself, of developing into a new organism ; such an organ is an asexual reproductive organ, and such cells 8,re asexually produced spores. In the latter case the reproductive organs are such that they do not singly give rise to cells capable, each by itself, of developing into a new org,anism. These are sexual repro-ductive organs. In some instanbes the sexual organ does not give rise to reproductive cells at all until it has received into itself rnore or less of the protoplasmic contents of another different, at least physiologically, sexual organ ; and the cells which it then produces are capable, each by itself, of developing iuto a new organism. In others the sexual organ produces reproductive cells without any such previous fusion of protoplasm, but the cells thus produced are incapable, each by itself, of developing into a new organism. Such cells are sexual reproductive cells. It is only by the fusion of two such cells, physiologically different, that a reproductive cell is formed which is capable of developing into a new organism. The fusion either of the protoplasmic. contents of two different sexual organs or of two different sexual cells constitutes the sexual process. It may take place, according to circumstances, either within the organs or, in those cases in which sexual cells are produced and are set free, externally to them. The resulting cells aro sexually produced spores.
In some exceptional cases the normal production, sexual or asexual, of spores does not take place, but the new organism is developed vegetatively from the parent. When the sexual production of spores is suppressed, the ease is one of apogamy ; when the asexual production of spores is suppressed, the case is ono , of apospory. The following are instances of apogamy : in certain Ferns (Pteris cretica, Aspidium faleatum) the prothallium produces no sexual organs, but the fern-plant rises vegetatively as a bud upon it. Apospory has been observed to occur in Mosses and in some Ferns (Athyrium Polystichum augulare) : Mosses a new plant may be developed vegetatively from the tissue of the sporogonium ; in Atbyrium the sporangium, instead of producing spores, develops into a fern-prothallium. In the Characew apospory appears to be the rule. Apogamy and apospory will be more fully discussed subsequently in connexion with the subject of alternation of generations.
Asexual Re-production. - Reproduction by means of asexually produced spores is common to nearly all families of plants. It is wanting, among the Alg, iu the Conjugat, the Fucacem, and the Characem ; among the Fungi, in a few Peronsporem (Pythium vexans, Arto-trogus), Ancylistes Closterii, in Aplanes Braunii, among the Saprolegniea5; and, among the Ascomycetes, in Ere-mascus, Sordaria (Hypocopra), Ascobolus furfuraceus, Pyranema (Peziza) confluens, Gynmoascus, the Collemacete, and most other Lichen-Fungi.
In the simplest case the spore is developed from a single cell of the plant, which surrounds itself with the characteristic thick wall. This occurs only in plants of low organization ; Nostoc and Bacillus are examples of it.
In other cases the contents of the cell undergo division, each portion of the protoplasm constituting a spore. Examples of this are afforded, among unicellular plants, by Yeast and Protococcus, and in multicellular plants by the Confervace2, the Ulvacea5, and some Floridem.
In this case each cell, the protoplasm of which divides to form spores, may be regarded as a, rudimentary reproductive organ of the nature of a sporangium. In more highly organized plants special organs are differen-tiated for the production of spores. In the majority of cases the special organ is a sporangium, that is, a hollow capsule in the interior of which the spores are developed. In the Thallophytes the sporangium is a single cell. In the Muscinew it is a multicellular capsule ; in Riccia, in which the structure of the capsule is simple, the whole of the internal cells give rise by division to spores ; in other Liverworts, and in the Mosses, in which the structure of the capsule becdmes progressively more and more complex, a portion only of the internal cells give rise to spores. In the Ferns, Equisetacete, and Lycopodiaccve the sporangium is multicellular, but simple in structure. This is true also in the Rhizocarpm and in the Ligulata) (Selaginella, Isoetes), but in these plants there is this peculiarity that there are sporangia of two kinds, - some, namely, which produce one, or a few, large spores, macrospores, and are hence termed macrosporangia, and others which give rise to a large number of small spores, microspores, and are hence termed microsporangia. The Phanerogams also bear two kinds of sporangia, which have received special names : - the macrosporangium, which produces only one mature spore, is termed the ovule ; the microsporangium, which produces a large number of microspores, is termed the pollen-sac. In some cases among the Fungi the spores are I not produced in the interior of a sporangium, but are t formed by abstriction. This occurs in some Mucorini, such as Chmtocladium, in the Ustilagine2, the Entomoph-thoreze, the Peronosporece, the Ascomycetes, the Rusts (Uredinem), and the Basidiomycetes.
These asexually produced reproductive cells are com monly spoken of sitnply as spores, but in many cases some addition has been made to the word, or an altogether different name is applied to them, in order to mark sorne.i peculiarity in their mode of origin, to indicate their order of development, or to assign them without periphrasis to a particular group of plants. Thus, as has been mentioned,' zoospores are motile spores unprovided, for a time at least, with a cell-wall; stylospores are spores which are developed, not in sporangia, but by abstriction as mentioned above ; tetraspores is the name given to the spores of the Floridece to denote the fact that four spores are produced by the division of the mother-cell. The uredospores of the Uredinem are those which are produced during the summer, whereas the teleutospores of these plants are thoso which are formed in the autumn, at the end of the period of growth. It was the custom, at ono time, to speak of the spores of Fungi as conidia ; and at the present time the macrospores and the microspores of Phanerogams are better known as embryo-sacs and pollen-grains respectively.
The organs which give rise to the asexually produced spores are usually not confined to a particular part of the plant in the Thallophytes. Instances of this do, however, occur among the Ascomycetous Fungi - namely, in the Pyrenomycetes. Here the production of the stylospores takes place in definite receptacles known as pycnidia. In the vascular plants (Pteridophyta, Phanerogams), the development of sporangia, speaking generally, is confined to the leaves. In many of the Pteridophyta the sporan-giferous leaves do not differ in appearance from the foliage-leaves ; but in other cases they undergo consider-able modification, as in the Equisetacece, Marsiliacem, some species of Lycopodium and Selaginella, and notably in the Phanerogams. In the Phanerogams the modification is so great that the sporangiferous leaves have received special names ; those which bear the microsporangia (pollen-sacs) are termed the stamens, and those which bear the macro-sporangia (ovules) are termed tho carpels. When the sporangiferous leaves are thus modified they are usually aggregated together, and such an aggregate of sporangi-ferous leaves constitutes a flower.
Sexual Reproduction. - In nearly all classes of plants : above the Protophyta spores are formed by a sexual process ; and in those in which no such process ca,n be detected its absence is due, not, as in the Protophyta, to 3 the fact that sexuality has not yet been developed, but to its gradual disappearance. The phenomena of sexual reproduction will be most intelligibly stated by tracing them in the different main divisions of the Vegetable Kingdom - the Algm, the Fungi, the Archegoniata, and the Pha-nerogams.
The protophytic Algm are reproduced by asexually developed spores, but in some forms an indication. is already given of the differentiation of these spores into sexual reproductive cells which takes place in the higher forms of the group. In Protococcus, for instance, zoospores are produced, but the zoospores are not all precisely similar. In some cases the protoplasm of the cell divides only once or twice, the result being the formation of two or four relatively large zoospores, macrazoospores ; in other cases the protoplasm divides a greater number of times so that a considerable number of relatively small zoospores, nzicrozoospores, are pro-duced. Functionally these zoospores aro alike ; they all come to rest, and form new Protocoeci. Amongst the Confervoid, which are more highly organized plants than the protophytic Algoe, we find forms, of which Ulothrix may be taken as the type, which likewise produce macrozoospores and rnicrozoospores in their cells. The macrozoospores of Ulothrix simply come to rest and germinate ; they are distinctly asexual spores. The inicrozoospores may also do this, but not unfrequently they coalesce in pairs ; the product of fusion, the zygospore, as it is tenned, then develops into a Ulothrix filament. This fusion of two similar reproductive cells - this conjugation, as it is termed - is one of the simplest forms of the sexual process ; the zygospore is then a sexually produced spore, and the two cells which conjugate to forin it are spoken of as gametes, - planogantetes when they possess cilia, aplanoganates when they do not.
Comparing Ulothrix with Protococcus, we see that in both the maerozoospores are asexual reproductive cells, whereas the micro-zoospores of Ulothrix exhibit an imperfect sexuality, inasmuch as they may germinate without previous conjugation. The piano-gametes of Ulothrix are, however, to be directly connected with the microzoospores of Protoeoccus; that is to say, the gametes are to be traced back to asexual spores. This is a point of fundamental importance.
Similarly, in Botrydium, one of the SiphoneT, there are two kinds of zoospores, some of which are asexual and others sexual ; the visible difference between them is, in this case, not one of size, but the gametes have two cilia and the zoospores only one.
In the conjugation which takes place in the above-mentioned. plants the gametes are quite similar in form and size, and take an equal part in the formation of the zygospore. The first indication of the differentiation of sexual gametes is afforded by Edo-carpus siliczelosus and Seytosiphon, belonging to the Phxosporea3. The zoospores of these plants are produced in well-defined sporangia, some of which are multilocular and others unilocular. Inasmuch as only the zoospores developed in the multilocular spomngia have been observed to conjugate, that is, to be gametes, those developed in the unilocular sporangia are probably asexual. This being so, the multilocular sporangia are to be regarded, not as mere sporangia, but as sexual organs (Gainetanyia) producing sexual reproductive cells. The process of conjugation of the gametes is, according to Berthold, as follows : - the gametes are at first quite similar iu every respect; some of them, however, soon withdraw their cilia and. come to rest, whereas others remain actively motile ; one of the still niotile gametes then coalesces with one which has come to rest to form a zygospore. The gametes in this case behave differently in the process of conjugation : the one is passive, the other active ; the former is to be regarded as the female, and the latter as the male reproductive cell. But even in this case the gametes, if they fail to conjugate, can germinate independently.
In Cutleria the sexual differentiation of the reproductive cells is more marked ; the male and female cells are developed in distinct sporangia, which may be termed respectively antheridia and oogonia, two male cells being formed in each antheridimn, and one female cell in each oogonimn ; the female cell is considerably larger than the male, but they are both planogametes, that is, conjugating cells which swim by means of cilia.; the fernale cell, however, soon withdraws its cilia. and comes to rest, and then conjugation takes place, with the formation of a zygospore, much in the same way as in Ectocarpus described above.
The next stage in the sexual differentiation of the Algw is to be found in such forms as Volvox, Vaucheria, CEdogonium, Fucus, and the Charace.• In these plants, as in Cutleria, the reproductive cells are entirely incapable of independent germination ; they have lost altogether that characteristic property of spores ; as in Cutleria also they are developed in two kinds of sporangia - antheridia and oogouia - and they are very different from each other in their form, size, and behaviour. In most cases the oogonium gives rise to a single relatively large cell, the oosphere, which is at no time provided with cilia, and is not set free from the oogonium ; in some of the Fucacem, there may, however, be two (Pelvetia), four (Ozothallia [Aseophyllump, or eight (Fucus) oospheres produced in each oogonium, and in all the Fueacem the oospheres are set free from the oogonium. The antheridium gives rise to a large number of small ciliated cells, the antherozoids, one of which subseqn ently fuses with the oosphere. The coalescence of two such highly differen-tiated sexual cells is termed fertilization, to distinguish it from the conjugation of similar sexual cells, and the product of fertiliza-tion is termed au oospore, to distinguish it from the product of conjugation, the zygospore. In these plants the antherozoids still retain the essential characters of planogatnetes, whereas the Oospheres have lost them.
But it is not in all Algm that sexual reproduction is effected by S. the fusion of well-defined sexual cells. In the Floridem the sexual r reproductive organs are well differentiated. The male organ is an t antheridium which produces antherozoids ; but these antherozoids I are peculiar in that they have no uilia, and aro surrounded by a cell-wall ; they are frequently, on this account, termed spermettia. The female organ is termed a procarpium or carpogonium ; it may consist of one or many cells, but in all eases it consists of a pro-jecting filament, the trichogyne, and a. more expanded basal portion. It is peculiar in that no well-defined oosphere exists within it. Fertilization is effected by the antherozoid (spermatitim) being passively brought into contact with tho trichogyne ; complete fusion then takes place, the contents of the sperruatium passing into the trichogyne ; the trichogyne now withers, and changes take place in the basal portion of the procarpium, one or more of the cells termed. carpogenous cells divide, and by a process of budding give rise to a cluster of cells which are capable of germinating, and 'pro-duce new plants ; these are termed e,arpospores. At the same time the cluster of spores frequently becomes surrounded by an up-growth of tissue ; the mature fructification is termed a cyslocazyz. From the female organ of the Floridem there are, then, formed, in consequence of fertilization, a number of reproductive cells, the carpospores, each of which corresponds to the oospore of plants like Fucus, Vaueheria, Sze. A more detailed comparison with Fucus makes this correspondence at once apparent. In Fucus the contents of the female organ (oogonium), which is a single cell, divide into eight oospheres, which are subsequently fertilized ; in the Floridem there is, before fertilization, no differentiated oosphere, but the organ, as a whole, is fertilized by the spermatium, and it is in this case after, and not before, fertilization, as in the case of Fueus, that a process of cell-formation takes place in the female organ ; hence the reproductive cells formed hy the fertilized female organ of the Floridem are at once fertile, and correspond to the fertilized oospheres (oospores) of Fucus.
The peculiarities of the sexual reproduction of some of the Floridew are of sufficient general interest to be mentioned here.
In the Corallineze, according to Solins-Laubach, the procarpia are produced several together in a conceptacle ; it is, however, only the central proearpia of the group which are capable of being fertilized, and the peripheral procarpia which produce carpospores. After the fertilization of the central procarpia, the earpogenous cells of the whole of the procarpia fuse together to form one large cell from the periphery of which the carpospores are produced by budding.
This physiolonical division of labour is more marked in Dud-resnaya, and a fPew other Floride. In these plants some of the procarpia are destitute of a trichogyne, hereas others possess that organ. The spermatia fertilize those procarpia which possess a tnchogyne, but these procarpia do not .produce carpospores ; but there grow out from them filaments which fertilize the procarpia destitute of a trichogyne, and these then give rise to carpospores.
The development of the carpospores in the Bangiace (Bangia, Porphyra) is peculiar. The earpogenons cell does not in this ease, as in other Floridese, produce spores by budding, but its proto-plasm divides into eight portions ; these are set free as naked masses of protoplasm, which move about for a time in an ameeboid manner and then come to rest and surround themselves with a cell-wall.
In some of the higher Algm, namely, in the Sphacelariew and in . the Laminariew, families belonging to the Plimosporem, no sexual process has been observed as yet ; but, as our knowledge of the life-history of these plants is imperfect, it cannot be definitely i stated at present that they are entirely asexual. , In the somewhat aberrant group of the Conjugat the sexual 1 process is peculiar. In the Desinidiew and the Mesocarpex it is ;, effected in this way, that two adjacent cells, belonging usually to different filaments, throw out corresponding lateral protuberances which meet, and, the intervening walls being absorbed, form a canal placing the cavities of the two cells in direct communication ; the protoplasm of each cell contracts, forming an aplanogamete, and travels into the canal, tvhere the two masses meet and fuse. This is deafly a process of conjugation, similar to that of plano-gametes, and. the product is likewise a single cell which is termed a zygospore. In the Zygnemese, of which Spirogyra is a familiar example, the process is slightly different. Here the protoplasm of one of the two conjugating organs contracts first and passes over into the cavity of the other, there to fuse with its protoplasm.
Turning now to the Fungi, we find that in the simplest forms (Schizomycetes, Sacchaiomycetes) there is no trace of sexual reproduction, whereas in the higher forms with some exceptions, sexual reproductive organs are present, th'ough they are in many eases functionless.
The lowest Fungi in which a sexual process has been observed are the Chytridiacem (Polyphagits Eagkne, the Mucorini, and the Entomoplithorete. In these, when it occurs, it takes the form of conjugation, with the production of zygospores. Conjugation is here effected in essentially the same manner as that described above for the conjugation among the Alpe, by the fusion of two similar sexual organs. Conjug,ation of planogametes has been observed. by Sorokin in Tetrachytrium and in Haplocystis, pro-bably belonging to the Chytridiacere, a fact of some interest as it is the only case of this form of the sexual process kuown in the Fungi.
Protomyces and the Ustilaginem a process takes place which appears to be of a sex-ual nature, resembling the conjugation of the Mucorini. Certain reproductive cells of an elongated form, termed sporidia, are produced, which become connected by a transverse canal so that they then resernble the letter H. No zygospore is formed, but the conjugated sporidia are its equivalent. The question of the sexual nature of this process is still under discus-sion, but it is made highly probable by the fact that, in all fully investigated cases, the sporidia aro incapa.ble of independent germination.
In the allied groups of the Peronosporete and Saprolegniete two kinds of sexual organs are present, male anti female, which corre-spond to the antheridia and oogonia of the Algre. The female organ is here also termed an oogonium, and, like that of the Algre, it may produce one oosphere (Peronosporere) or many (most Saprolegniete). The male organ is also usually termed an anthe-ridium, but it is sometimes spoken of as a pollinodium on account of the mode in which it effects the fertilization of the oosphere. In one of the plants allied to these gronps - namely, in illonoble-pharis sphEerica, according to Cornu - the protoplasm of the autheridium becomes differentiated into motile antherozoids ; this is the only case of the kind known among the Fungi. In the other members of these groups in which fertilization takes place the antheridium lies in contact with the oogonium, and produces at its apex a delicate tubular outgrowth, which bores its way through the wall of the oogonium and comes into contact withan oosphere ; the tube then opens, and protoplasm from the antheridium passes through into the oosphere and fuses with it to form an oospore.
In the Ascomycetes sexual organs are very commonly present, if but it is only in a few cases that a sexual process has been observed - actually to take place. The sexual organs differ considerably in their form in the different genera. In some (e.g., Gymnoascus, Eremascus, Eurotium, Penicillium) the sexual organs are similar ; they are unicellular or multicellular hypliT, but in some the female organ, termed in the Ascomycetes the ascogonium or carpo-gonium, may be distinguished from the male organ, which is a pollinodial antheridium like that of the Peronosporem, in that it is wound into a close spiral. In others (e.g., Erysiphete, Ascobolus, Pyronema [Peziza] confluens) the sexual organs are readily dis-tinguishable. In the Erysiphez and in Pyronema the ascogonium is a single relatively large ovoid cell ; that of Pyronerna produces a delicate tubular outgrowth, the trichogyne ; the antheridium also is unicellular, but it is more slender. In Aseobolus the ascogonium consists of a row of five or six relatively large cells ; the antheridium is a slender multicellular hyplia. In all these eases the sexual organs are developed in such close proximity to each other that they .come into contact. In other Ascomyeetes I oelonging to the Discomycetous Lichens (Collema, Synechoblastus, Leptogium, Physma), and to the Pyrenomycetes (Polystigma), the antheridial filaments, termed sterigmata, are developed at a dis-tance from the female organ in separate receptacles, sperotogonia. In this case the formation of male cells is a necessity. Accordingly cells, termed spermatia, are produced from the sterigmata by abstriction, which resemble the male cells of the Florid= in that they are non-motile and have a cell-wall.
With regard to the process of fertilization, it must be premised that in no Ascomycete is the protoplasm of the ascogoninm ever differentiated into an oosphere. When the sexual organs are ascogonia and pollinodial antheridia, fertilization takes place by the fusion of the undifferentiated protoplasmic contents of the two organs, a mode of fertilization which recalls the conjugation in the Mucorini, and in the Conjugatre aniong the Algre. This has only been observed to take place in Eremaseus and in Pyronema; in the latter the antheridium applies its apex to the trichogyne of the ascogonium, the intervening walls are absorbed, and the pro-toplasm of the two organs coalesces. Although this process of fertilization resembles the conjugation occurring in the Mucorini, the product is very different. The product of conjug,ation in the Mucorini is a single cell, the zygospore ; the product of fertiliza-tion in the Ascomycetes is a -"umber of cells, termed aseospores or carpospores. Thus in Eremascus the product of fertilization is a unicellular capsule, the ascii.; in which eight ascospores are formed. In Pyronema the fertilized aseogonium enlarges and • givea rise to a number of outgrowths which produce asci ; at the same time a number of hyphre grow up from below around the developing asci, some of which produce delicate filaments, termed paraphyses, which lie amongst the asci, whereas others form an investing wall. The result is the formation of a fructification, termed an apoaccizon. Within each ascus eight spores ais formed.
In those Ascomyeetes which have spermatia fertilization is effected, as in the Floridere among the Algte, by the fusion of a spermatium with the trichogyne. The result is the same as in Pyronema : the fertilized ascogoninm gives rise to hyplite which bear asci, and these, together with sterile hyphre, form a fructi-fication.
It is of interest to note the similarity between the products of fertilization in these Ascornycetes and in the Floridete. In both cases the female organ produces no differentiated oosphere, and in both cases the product of its fertilization is a many-spored fructi-fication. It was pointed out, in speaking of the Floridere, that each carpospore is the equivalent of a fertilized oosphere (oospore) ; this holds good also with regard to the carpospores (ascospores) of the Ascomycetes.
It may be that a similar sexual process takes place in the other forms mentioned above, viz., the Erysiphem, Penicillium, Sordaria, &c., but it has not been observed ; in any case, the ascogonium in all these plants gives rise to asei and ascospores, and a more or less complex fructification is produced. But th,ere is also some ground for believing that in some at least of these eases the sexual organs, though morphologically differentiated, are functionless. For there are clear indications of sexual degeneration in the Ascomycctes. In some cases, for instance (e.g., Chxtornium, Melanospora), no antheridium can be distinguished, but the ascogonium eventually produces asci nevertheless. In others (e.g., Xylaria, as far as known at present) no male organ is produced, but there is an ascogonium which does not, however, give rise to asci ; the asci, as well as the rest of the fructification, arise from the vegetative hyphT. In others, again (e.g., Claviceps, Cordiceps, Pleospora), all trace of the sexual organs has disappeared, but a fructification containing ascospores is produced, as in Xylaria, from the vegeta-tive hyph,. In others, finally, no ascospores are known, the only reproductive cells being the characteristic asexually produced stylospores.
In the remaining groups of Fungi, the Uredinem and the Basidio-mycetes, no sexual reproduction is known. In the Basidiornycetes no kind of sexual organ has been discovered. In the UredineT spermatia are commonly produced, as in the Ascornycetes mentioned above, but no female organ is known ; however, fructifications termed wcidiet are in some cases developed. These resemble somewhat those of the Ascomycetes, but differ in that here the spores (tecidiospores) are formed by abstriction, and not in asci as in the Ascornycetes.
It may be suggested that the ascosporcs of the asexual Ascomy-cetes and the mcidiospores of the Uredinere should not be included in an account of the sexual reproduction of the Fungi. It is true that these spores are asexually produced, but their evident homo-logy with the sexually produced spores makes it inconvenient to treat of them apart. They differ from the sexually produced spores in that they are developed apogamously.
Under the name Archegoniata we may conveniently group together the Museinete and the Vascular Cryptogams (Plerido-phyta). The sexual organs, as also the process ot fertilization, are essentially the same throughout. The female organ produces a single oosphere, and is termed an areltegonitint ; it is essentially similar to the oogonium of the Thallophytes, the only difference being that, whereas the archegoninm is multicellular, the oogonium is unicellular. The male organ, here also termed the antheridium, is likewise rnulticellular, and gives rise to a larger or smaller nurnber of motile antherozoids. Fertilization is effected by the fusion of an antherozoid with the oosphere, which then clothes itself with a cell-wall and becomes an oospore.
In the Phanerogams the sexual organs are essentially of the nature of archegonia and of antheridia, but they are somewhat modified and are called by other names. The female organ of the Gymnosperms, termed a corptsculum, closely resembles the arch°. gonium of the Archegoniata, and produces a single oosphere. In the Angiosperms, the female organ is much reduced, consisting only of three cells, one of which is the oosphere, the other two being the synergidx which assist in the process of fertilization ; the organ is termed the egg-apparatus. The male organ in the Phanerogams is a unicellular filament termed the pollen-tube ; its protoplasm does not undergo differentiation into antherozoids. The sexual organs of the Phanerogams recall those of the Perono-sporete and the Saprolegniett ; in both cases tho female organ pro-duces an oosphere, and in neither does the protoplasm of the antheridium produce antherozoids. The process of fertilization will be described subsequently.
Physiology ni Reproduction. - From the fact that in even the most highly organized plants an isolated portion of one member is capable of producing, not merely a member like itself, but other members also, so that a new individual is constituted, it is clear that the protoplasm of plants is imperfectly differentiated physiologically. Never-theless all plants produce cells to which the work of repro-duction is especially assigned. It is of interest to recall the fact that a suppression of spore-formation, either asexual or sexual, may occur, and vegetative multiplication be reverted to, as in aposporous and some apogamous plants.
It has been shown above that the reproductive cells of plants are of two kinds - those, namely, which are indivi-dually capable, and those which are-individually incapable, of giving rise to a new organism ; the former are the asexual, the latter the sexual reproductive cells. It has also been indicated that the latter are to be regarded as derivatives of the former, a point which may now be somewhat more fully established. It was pointed out, namely, that the gametes of Ulothrix will, if they fail to conjugate, germinate independently; the sexual differen-tiation of these gametes is clearly imperfect, and they differ but little from asexual zoospores. The same thing has been observed in Botrydium, and this is a specially interesting case inasmuch as it throws some light upon the conditions which determine sexual differentiation of the reproductive cells in these lowly organized plants. It has been ascertained that the nature of the cells produced from the resting-spore, in the manner described above, depends upon the age of the spore producing them : when the spore is young, the cells produced by it are sexual gametes; if they fail to conjugate they perish ; when the spore is old, the cells produced by it are entirely asexual zoospores ; tbey never conjugate, but each by itself gives rise to a new indi-vidual. The imperfect sexual differentiation of the gametes has also been observed in Ectocarpus ; if they fail to con-jugate they germinate independently. The occurrence of this in Ectocarpus is rather surprising when it is remem-bered that the gametes of this plant are to some extent sexually differentiated as male and female (see above).
From these cases in which the typically sexual repro-ductive cells still possess the properties of asexual spores we pass to others, like Acetabularia, in which they have entirely lost these properties. The planogametes of this plant are definitely sexual ; but they are quite similar, as far as external appearance goes, to each other ; there is no perceptible distinction between male and female cells. This is the case also in the Mesocarpem and the Desmidiem among the Conjugatm ; here the non-ciliated conjugating masses of protoplasm (aplanogametes) are externally similar • and take an equal part in the sexual process. In Cutleria r the pIanogametes, and in the Zygnemem the aplanogametes, give indications of further sexual differentiations ; in Cut-leria the female gamete is much larger than the male and comes sooner to rest ; in the Zygnemem the one aplanoga-mete passes over into the cell producing the other apla.no-gamete and fuses with it ; the former is to be regarded as male, the latter as female. Finally, in the oosporous Alg, in the Muscinew, and in the Pteridophyta the two cells are quite distinct in form, size, and behaviour ; the male cell (antherozoid) alone retains the character of a planogamete, the female (oosphere) is non-motile and is many times larger than the antherozoid. In this series the gradual differentiation of the highly differentiated sexual cells from asexual cells can be clearly traced.
If the sexual reproductive cells are to be traced back to araxual spores, then the organs which produce the sexual reproductive cells are also to be trned back to those which produce the asexual spores, namely, the sporangia ; the most highly differentiated sexual organ - the antheridium, the. oogonium, the archegonium, the carpogonium - is derived from the sporangium.
The question now arises as to the nature of the differ-ence between sexual and asexual reproductive cells. It would appear that the former are in some way incomplete, that something is lacking to them which the latter possess, and that this lack is supplied in the sexual process. In many cases facts have been observed in connexion witb the development of the sexual cells which indicate that they are thus incomplete. In Acetabularia the whole of the protoplasm of the gametangium is not used up in the formation of the gametes, and, in the Peronosporem only a portion of the protoplasm of the oogonium forms the oosphere ; the remainder is simply the periplasm. In , Vaucheria and other Algm a mass of protoplasm escapes from the oogonium when it opens. In other cases a pro-cess of cell-division has been observed to accompany the formation of the oosphere which recalls the production of the " polar bodies " in the developing eggs of animals. In the Archegoniata the central cell of the archegonium does not directly give rise to the egg, but a portion, the ventral canal-cell, is first cut off ; this takes place also in the cor-pusculnm of most Gymnosperms. Similarly in the develop-ment of antherozoids, the whole of the protoplasm of the mother-cell is never used up in their formation.
In the germinating mierospore of most of the Heterosporous Vascular Cryptog,arns and of the Phanerogams a process of cell-division takes place which Strasburger interprets as the formation of a polar body. The protoplasm of the microspore undergoes division so that two cells are formed, which may be distinguished as the vegetative and the generative, the former beinF much smaller than the latter in the Vascular Cryptogams and in the Gymno-sperms, whereas in the Angiosperms the converse is the case ; uhually the separation of the ti,vo cells is permanent, but in most Angiosperms it is transitory, the only permanent indication of the cell-division being the presence of two nuclei in the pollen-grain; in some Gyrnnosperms two or three more vegetative cells may be cut off from the generative cell. The antheridinm is in all eases formed from the generative cell. These vegetative cells Stras-burger regards as o'f the nature of polar bodies. The nucleus of the generative cell undergoes division, to form in the Ileterosporous Vascular Cryptogams the nuclei of the mother-cells of the anthero-zoids, and in the Phanerogams the nuclei which take part in the sexual process, as will be more fully described below.
The assumption of the incompleteness of the sexual cells may be extended to those sexual reproductive organs which, like the procarpia of the Floridem, the pollinodial antheridia of the Peronosporem, the ascogonium and pol-linodial antheridia of the sexual Ascomycetes, do not give rise to differentiated sexual cells.
There is reason to believe that the sexual reproductive cells are spores which, by tbe loss of certain of their con-stituents, have undergone sexual differentiation, and that those sexual organs which directly take part in the sexual process without the intervention of reproductive cells are sporangia which have undergone sexual differentiation in the same way. This is finally proved by the fact that in cases in which the normal phenomena. of sexual differen-tiation do not take place the reproductiVe cells can ger-minate without fertilization, and the female sexual organ can produce, without fertilization, cells capable of germina-tion. These cases are examples of that form of apogamy which is known as parthenogenesis. Parthenogenesis in plants producingdifferentiated sexual cells hasbeen observed in the Mucorini, the Entomoplithorere, and the Saproleg-niem among the Fungi, and in Chara crinita among the Algm. In some Mucorini (occasionally in Absidia septata, A. capillata, Hue° fusiger, Sporodinia, always in illucor tennis) and in some Entomophthoreaa, namely, the conjugat-ing hyphm remained closed, and the protoplasmic contents of each surrounds itself with a cell-wall, the cells thus pro-duced being quite similar to the normal zygospore ; these cells are termed azygospores. In the Saprolegniem and in Chant crinita the oospheres behave like oospores and germinate in the same manner. The details of the development of these asexual sexual reproductive cells has been fully investig,ated by De Bary in the case of the oosphere,s of the Saprolegniem. In this case that differen-tiation of the protoplasm of the oo,gonium into ooplasm. and periplasm which has been described above as occurring in the closely allied and completely sexual Peronosporem does not take place, but the whole is used in the formation of the oospheres. It is quite clear also that in the Sapro-legniem no sexual process takes place. In some species of Saprolegnia the antheridia are altogether absent, in others they are rudimentary, and even in those Saprolegniem (some species of Saprolegnia, Achlya, Aphanomyces) in which the antheridia are well-developed they remained closed. No case is known of male parthenogenesis, that is, of the development of an individual from a male repro-ductive cell, among plants in which the sexual differentia-tion of the reproductive cells is well-marked, but there are instances of the kind in more lowly organized plants. Thus in the Mucorini, mentioned above, one of the azygo-spores produced may be regarded as belonging to a male organ ; again, in Ectocarpus the male planogamete germi-nates independently as well as the female. Among plants the sexual organs of which do not produce specialized re-productive cells instances of parthenogenesis also occur. Thus the spores (ascospores) which are produced by the female organ (ascogonium) of those Ascomycetes, such as the Frysiphem, Penicillium, Melanosoora, Xylaria, in which the male organ is functionless or absent can only be regarded as being parthenogenetically produced.
Admitting, then, that sexual differ from asexual reproductive cells in the lack of something which the latter possess, and which was thrown off either from the former or from the organs which produce them, we may go on to inquire what this something may be. Our in-forMation on this point is very scanty, but Strasburger's views throw some light upon it. He considers, namely, • that the formative processes of the cell are regulated by the hyaline plasma of the nucleus - the nucleo-idioplasma, as he terms it ; the richer the nucleus is in this substance the more capable is the cell of producing new cells. The asexual reproductive cells are then cells the nuclei of which are especially rich in this substance. He considers that the differences between asexual and sexual reproductive cells is quantitative and not qualitative - that the nuclei of the former are rich in nucleo-idioplasma, those of the latter poor, either originally or by the throwing off of part of their substance in the mode described above. In en-deavouring to account for the further differentiation of sexual reproductive cells into male and female, it seems natural to suggest that the respective nuclei have under-gone a qualitative differentiation, and that in the sexual process the qualitative, as well as the quantitative, incompleteness of each is made good. Strasburger is, however, strongly of opinion that this is not so, but that the difference is purely quantitative. But it must be pointed out that, according to this view, there is no reason why a sexual process should not take place, either between two male cells or organs, or between two female cells or organs, a possibility which is never realized, nor is it possible to account for the fact to be subsequently dis-cussed that in many cases a sexual process cannot take place between sexual cells or organs of a closely-allied origin.
The nature of the sexual process will be better under-stood. by .a detailed description of it in some particular case. The following is a brief account of the results of Strasburger's observations on the process of fertilization in Angiosperms.
At tho period of the dehiscence of the anther, the protoplasm of the pollen-grain undergoes division into two cells - a smaller, the - generative cell, and a larger, the vegetative cell. At the time of pollination, when the pollen-grain is mature, it is usually- the ease that the only persisting indication of the previous cell-division is the presence of two nuclei in tho protoplasm. In many cases the two nuclei were found to differ qualitatively, inasmuch as tho generative nucleus stained. readily when treated with carmine, picro-carmine, methyl-green, &c., whereas the vegetative nucleus stained imperfectly or not at all. On the formation of the pollen-tube, tho generative nucleus, and sometimes the vegetative nucleus also, is carried down into it, and the former then undergoes division into two ; occasionally one of the new nuclei divides again. The pollen-tube grows down the style, enters the ovary, and is directed to the micropyle of au ovule. In the ovule the egg-apparatus lies at the micropylar end of the embryu-sac, and the delicate wall of the embryo-sac covering it undergoes absorption. In its further growth the pollen-tube comes into contact with the synergidte ; in some cases its growth now stops, and. a portion of its protoplasm, with one or both of the generative nuclei, passes through the mucilaginous wall at the apex of the tube, wtthout leaving any opening behind it, and travels to the oosphere ; in other cases the pollen-tube grows between the synergile to the oosphere, and at once pours a portion of its protoplasm with a generative nucleus into it. This is followed by the appearance of a. second nucleus in the oospliere (filo male pronucleus), which is the generative nucleus derived from the pollen-tube, whicli now travels towards the nucleus of the oosphere (female pronu'eleus) and fuses with it ; when the two pronuclei have each a nucleolus these also eventually fuse. Fertilization is now complete.
The synergidre take no direct share in the process of fertilization, but becorne disorganized ; their disorganization usually begins when the pollen-tube first comes into contact with them. They serve merely to direct the pollen-tube or its escaped contents to the oosphere and to provide nourishment for them.
The fusion of the male and female pronuclei has been observed also in the oosphere of the Gymnosperms, and in the conjugation of Spirogyra.
Since plants are so commonly able to reproduce vege-tatively, the question arises as to the necessity of the production, either sexually or asexually, of spores. The biological importance of these cells is very great. They are capable, namely, of retaining their vitality under external conditions, such as long drought, absence of food, extremes of heat and cold, which would prove fatal to the plant, and they therefore are essential to the maintenance of the species. Further they are of importance in the distribution of the species ; they are light, readily transportable by wind or water, in some cases themselves actively motile. In this way they serve to prevent that close aggregation ot individuals of the same species which would result from a continued vegetative multiplication, and would prove in-jurious to the species. In the case of Phanerogams, in which the macrospore is not set free from its sporangium, the same ends are obtained by the production of seeds.
The further question now arises as to the importance of the sexual process.
It appears that, if any given species, at least among the higher plants, is reproduced through a long series of generations in a non-sexual manner, the individuals tend to degenerate, and the original well-developed form can only be reattained by the formation of a sexually produced spore. This result is to some extent realized by the fusion of two sexual cells or organs belonging to the same indi-vidual - that is, by self-fertilization - but more completely when the fusion takes place between sexual cells or organs belonging to distinct individuals - that is, by cross-fertiliza-tion. In some plants, as in the Peronosporem and in those sexual Ascomyeetes which have pollinodial antheridia, self-fertilization alone is possible ; this is also the case in certain Phanerogams in which the arrangements are such (notably in cleistogamous flowers) that only pollen from its own anthers can reach the stigma of the flower. In most cases, however, the conditions under which the sexual process is carried on, such as the formation of free-swimming gametes and antherozoids, and of spermatia and pollen-grains which can be readily conveyed from place to place, are such as to favour the occurrence of cross-fertilization. In some there are special arrangements for the attainment of this end, the most general of which is dicecism, that is, the produc-tion of the male and female organs by distinct organisms. Thus in certain Fucacem (Fucus veliculosus, E nodosus, F. serratus, Hintantitalia lorea) some individuals bear only antheridia, and others only oogonia; among the Muscinem the plants are frequently either male or female; in the Isosporous Vascular Cryptogams the prothallia are usually hermaphrodite ; but exclusively male and female prothallia occur not unfrequently in the Filice and as a rule in the Equisetacem. In the FIeterosporous Vascular Cryptogams dicecism is brought about in a somewhat different manner: these plants have, as mentioned above, two kinds of spores, macrospores and microspores; the former on germination always give rise to a female (archegoniate) prothallium, the latter to a male (antheridial) prothallium; hence the male and female organs are necessarily borne on distinct organ-isms. In some Phanerogams even the two kinds of spores are produced by separate individuals, the flowers of the one producing pollen-grains (microspores) but no embryo-sacs, those of the other producing embryo-sacs (macrospores) but no pollen-grains. More special arrangements are to be found in the flowers of Phanerogams for ensuring cross-fertiliza-tion, and preventing self-fertilization, e.g., the development of highly-coloured perianth-leaves and the secretion of nectar to attract insects, dichogamy, heterostylism, &c.; but it is impossible to do more than mention these here.
But besides these obvious structural arrangements for ensuring cross-fertilization and preventing self-fertilization there are in some cases imperceptible physiological con-ditions which even more certainly lead to the same results. It appears, namely, that in such cases no sexual process can take place between reproductive cells or organs of closely-allied origin. Thus, among the Algm, in Ecto-carpus and in Acetabularia, conjugation only takes place between planogametes derived from distinct gametangia, and in Dasycladus it only takes place between piano-gametes derived from gametangia borne by different indi-viduals. Again, in many Phanerogams, as Darwin has shown, the pollen of one flower is quite incapable of fertilizing the oospheres of its own ovules, and the pollen-grains from another flower borne by the same plant is but slightly, if at all, more potent. The pollen from the flower of another individual of the same species is potent, and this the more so the wider the difference between the individuals ; the pollen from an individual of a different variety is more potent than that from an indi-vidual of the same variety.
The effect of the sexual process is not necessarily con-fined to the cells or organs which directly take part in it ; not infrequently it makes itself felt in adjoining organs, stimulating them to active growth, giving rise to the formation of a fruit or fructification. Thus in the Mu-corini an outgrowth of filaments, forming an incomplete or complete (Mortierella) investment to the zygospore, takes place from the sexual organs after conjugation ; in Coleochmte the oogonium becomes surrounded, after the fertilization of the oosphere, by an investment formed by outgrowths from the adjacent vegetative cells ; a cellular investment is formed in the same way round the fertilized procarpium in most Floridem, and round the fertilized ascogonium in the Ascomycetes. The most familiar case of fruit-formation is that occurring in the Phanerogams ; here in many instances the carpels, in some the floral leaves of the perianth, and in some the floral receptacle (torus) grow actively after the fertilization of the oospheres has taken place, giving rise to a mass of succulent parenchymatous tissue. In the Orchideve the develop-ment of the ovules does not take place at all until the flower has been pollinated.
Germination of the Spores and Alternation of Generations. - The spores of plants may either germinate immediately on their production, or they may undergo a. longer or shorter period of quiescence ; those which are destined for immediate germination have, as described above, a thin cell-wall, whereas those which are capable of undergoing a period of quiescence have a thick cell-wall. In some cases the spores are incapable of immediate germination, notably sexually produced spores; for instance, among the Algae, immediate germination is only known to take place in the case of the zygospores of Botrydium and Ectocarpus, and of the oospore of Fucus ; among the Fungi the zygospores of the Mucorini and the oospores of the Peronosporem and Saprolegniew pass through a period of quiescence.
The mode of germination is not always the same. In rnost cases the spore gives rise directly to a new organistn, either by protruding one or more filamentous outgrowths, or by the division of its protoplasm to form the tissue of the embryo. In some cases the spore behaves like a reproductive organ ; from its protoplasm are formed a larger or smaller number of cells, either motile or non-motile, which are set free. In Acetabularia, and under certain circumstances in Botrydium, the asexually pro-duced spore behaves like a sexual reproductive organ (gametangium), giving rise to a number of planogametes; similarly the spore of Protomyces produces within itself a number of conjugating sporidia. In some Peronosporem (always in Cystopus ; occasionally, according to circum-stances, in Pythium, Phytophthora, and Peronospora) the asexually produced spore behaves like a sporangium, and gives rise to a number of zoospores from each of which a new individual is developed. This happens occasionally also in sexually produced spores. Among the Fungi, the formation of zoospores in the oospore occurs in various species of Peronosporem and Saprolegnieze. Among the Algm, zoospores are formed in the zygospores of Pandorina and Ulothrix, and in the oospores of CUdogonium and Sphzeroplea. Cases of a similar kind are known also in the Phanerogams ; thus in some Coniferm, and notably in the Gnetaceous Epltedra altissima, a process of cell-forma-tion goes on in the oospore, resulting in the formation of a a larger or smaller number of cells from each of which an embryo plant is developed. These cases, in which, namely, the spore, whether sexually or asexually produced, gives rise to a number of cells, each of which is capable, by itself, of developing into a new individual, are instances of what is known as polyembryony.
In some cases the cells formed in the sexually produced spore do not each give rise to a new individual ; this obtains in the Hydro-dictyew. In Hydrodielyon utrieulatum the protoplasm of the zygospore gives rise to two or four large zoospores which eventu-ally come to rest and remain quiescent for several months ; these resting spores are termed, on account of their form, polyhedra. On germination, the protoplasm of the polyhedron gives rise to a number of small zoospores, thc endospore protruding as a delicate vesicle, within which the zoospores are in active movement ; the zoospores eventually come to rest, without escaping from the en-dospore, and arrange themselves so as to forro the meshes of a small sac-like net, which is a young Hydrodictyon ; the endospore is then disorganized, and the young net is set free as an independ-ent ccenobium.
Occasionally it happens that a portion only of the spore gives rise to the embryo. This is the case in the sexually produced spores (oospores) of tbe Characeee, and in those of the Coniferaz ; in Selaginella and in the Angiospermous Phanerogams one-half of the oospore gives rise to a filamentous structure, the suspensor, the other half to the main body of the embryo.
It is very commonly the case that the spore, on germina-tion, gives rise to an organism unlike that by which the spore was produced. In a Moss, for example, the asexu-ally produced spores are developed by an organism, the sporogonium, consisting typically of a longer or shorter ! stalk, the seta, bearing a, capsule (theca) which produces the spores. When one of these spores germinates, it does not give rise to another sporogonium, but to an incon-spicuous, usually filamentous, structure, the protonem(4 sporogonium. There is thus in the life-history of a Moss a renular alternation of a sexual with an asexual genera-tion ; the former may be conveniently termed the oophore, the latter the sporophore ; the asexually produced spore always gives rise to the oophore (moss-plant), the sexually produced spore (oospore) of the moss-plant always gives rise to the sporophore (sporogonium).
This kind of life-history is not peculiar to the Muscinem, but it can be traced, more or less clearly, in all the sively female, derived respectively from the microspores and the macrospores.
Such a life-history can be stated generally in the followproduced asexually, in the other sexually ; the sexual generation (oophore) springs from the asexually produced spore, and gives rise to the sexually produced spore (oospore) from which the asexual generation (sporophore) is developed. It is of interest to note that in most cases the organism developed from the sexually produced spore is much more highly organized than that developed from the asexually produced spore.
A peculiarity of the macrospores of the Phanerogams is worthy of mention here, as it leads to the production of that structure, the seed, which is characteristic of Phanerogams ; the production of a. seed constitutes, in fact, the only real and constant distinction between Phanerogams and Cryptogams.
As a rule the asexually produced spores of plants become free from the sporangium in which they have been formed. In Phanerogams this is true only of the microspores (pollen-grains), the macrosporcs (embryo-sacs) remaining permanently enclosed in the sporangia Onucellus) producing them. This being the case, their germination, i.e., the formation of endosperm, must take place within them whilst enclosed in the spomngiuzn ; and, further, the formation of tho female reproductive organs and the development of the embryo must take place under these circumstances also. The result is the production of a, seed. In a typical seed three genemtions of the plant are represented ; they are as follows ; - sporangial tissue belonging to the parent sporophore - perisperm ; tissue belonging to the oophore - endosperm ; the new sporophore - embryo.
In the seed the development of the new asexual plant proceeds to a certain limit. "When this is reached the development ceases, and it is only when the seed is placed under favourable conditions that the further development of the embryo, i.e., the germination of the seed, can take place.
It has now been shown that in the life-history of the Muscinem and in plants above them in the vegetable kingdom there is a regular alternation of generations, and the question now arises as to how far this is true of the life-history of plants lower than the Mosses, that is, in the Thallophytes. It is clear that no such alternation can take place in the life-history of those which are known to reproduce only either sexually or asexually, nor in that of those individuals which produce spores both sexually and asexually, either simultaneously or at different times.
The following are instances of the above-mentioned possible cases : - Thallophytes reproduced only by asexzualy produced spores : - Algx Cyanophycere or Phyeoehromaeem ; Protococcaeete ; Sphacelartm and Lanzinariem so far as known at present.
Fungi : Schizomyeetes ; Saceharomyeetes ; Myxoznyeetes ; some Chytridiem ; probably many Mucorini ; a few Peronosporeaa (probably Ph,ytophthora ivfestans and Pythium intermedium); some Ascomycetes and Ure-dinem ; Basidiomycetes.
Thallophytcs reproduced only by sexuall,y produced spores : - Algec: Conjugatre ; Fueacem ; Sphteroplem. The ease of the Characem will be subsequently discussed.
Fungi : a few Peronosporem (Pythium vexans, Artotrorms) ; Ancylistes Closterii ; Aplanes Braunii among the Sbapro-legniem ; some Ascomycetes, Eremascus, Sordaria (Hypoeopra), Ascobolus furfuracezts, Pyronema confluens, Gymnoascus, Collemacere and other Lichen-Fungi ; some Uredinem in which only mcidiospores are known.
Thallophytes in which the same form produces spores both asexually awl sexually : - Algx Vaucheria, Hydrodietyon, Ulothrix, (Edogonium, some Floridem (e.g., Polysiphonia variegata).
Fungi: Mueorini ; most Peronosporem and Saprolecmiem ; Monoblepharis ; among the Ascom.yeetes, the bErysi-phem, Eurotium, Penicillium, Nectria ; some Uredinete (Uromyces appendiculatzts, U. Behenis, U. Scrophularix, CT. Ccstri, Puccinia Berberidis).
In some of the Algm, as in the Volvocinew, in the life-history of which distinct sexual and asexual forms occur, no alternation of generations can be traced, since there is no certainty as to the nature of the form arising from any given spore ; the form developed from the asexually pro. duced spore is not, as in the typical life-history of the Mcss, necessarily sexual, nor is the individual produced from the sexually produced spore necessarily asexual. But in the life-history of some others an alternation of genera-tions is traceable. Thus in the Siphonaceous Acetabularia the plant produces spores; these, as mentioned above, behave on germination as gametangia ; the gametes con-jugate to form a zygospore, and from the zygospore the asexual Acetabularia springs. Here there is a distinct and regular alternation of generations ; the Acetabularia-plant is the asexual generation (sporophore), the garnet-angia alone representing the sexual generation (oophore). The life-history of Botrydium, another Siphonaceous Alga, is essentially the same as that of Acetabularia, but it is frequently less regular ; thus the resting-spores, instead of producing gametes, may directly germinate to form a Botrydium-plant, a process which is clearly a case of apogamy ; and, further, the Botrydium-plant does not necessarily produce resting-spores, but may produce zoospores by which it is directly reproduced. In Coleo-chmte, too, alternation of generations is indicated. The oospore produces by division a small individual which is always asexual, giving rise to zoospores which likewise produce asexual individuals; this asexual reproduction may continue through a number of generations until eventually a sexual individual is developed. In Coleo-chmte it is only in the case of the sexually produced spore that the nature of the resulting organism is known ; it always gives rise to an asexual form, whereas the asexually produced spores give rise to an individual which may be either sexual or asexual. In the Characem the oospore always gives rise to an imperfectly developed form, the proembryo, which is to be regarded as the sporophore ; however, it never produces spores, but gives rise to the sex-ual Chara-plant (oophore) vegetatively by budding.
The study of the life-history of the Fungi is attended with considerable difficulty, partly on account of the fact that in many cases the development of the sexual organs is dependent upon a combination of external circumstances which may but rarely present itself, and partly on account of there being frequently a great difference in habit between the sexual and asexual forms of the same plant, a difference which is sometimes accentuated, in parasitic Fungi, by the occurrence of the two forms on different plants as hosts (hetercecism). But there are Fungi in the life-history of which alternation of generations has been ascertained. Before entering upon an account of these, it must be stated that the term "sexually produced spore " will be applied not only to those the formation of which is known to be preceded by a sexual process, but also to those which are formed probably or actually without a sexual process - in a word, apogamously - but which may be considered, as pointed out above, to be homologous with those which are actually sexually produced.
In Alucor Mucedo and Pkycomyces nitens among the Mucorini, for instance, the zygospore gives rise on germination to an imperfectly developed individual (pro-mycelium) which is entirely asexual and produces spores; one of these spores, in turn, gives rise to an individual which produces spores asexually but may also bear sexual r6productive organs. Essentially the same life-history may be traced in certain Peronosporem (Phytopktkora omnivora, Pytkium prolzferum). In these cases the form developed from the sexually produced spore is always asexual, where-as that derived from the asexually produced spore may be sexual, but it always produces spores asexually ; hence there is not a strict alternation of an asexual and a sexual generation. In others the alternation is complete. In the Ustilaginece, for instance, the asexually produced spore gives rise to an imperfectly developed mycelium (promycelium), which is the sexual generation it produces the sporidia, which conjugate in pairs, an'd from the product of conjugation springs the mycelium, which bears aseivally produced spores.
Essentially the same life-history has been traced in some Ascomycetes and Uredinem. In Cla.viceps (Ascomy-cete) the sexually produced spore (ascospore) gives rise to an asexual form, long regarded as a distinct genus under the name of Sphacelia, from the spores of which the sexual form is reproduced. In Sderotinia (Peziza) Fuckeliana (Ascomycete), a similar regular alternation of generations occasionally but not always occurs; the ascospore may give rise to an asexual form, long known as Botrytis cinerea, and when it does so the alternation of generations is complete; but it may give rise to another sexual genera-tion, in which case no- alternation takes place. In Polystigma (Ascomycete), the ascospore gives rise to a promycelium which bears sporidia, and these sporidia give rise to the sexual form. In Endophyllum (Uredinem) the life-history is precisely the same as that of Polystigma: the promycelium is the sporophore generation, the myce-lium developed from the sporidium the oophore. In other Uredinere the life-history is slightly modified in that asex-ually produced spares of at least two kinds make their appearance. The sexually produced spore (mcidiospore) gives rise to a mycelium which, in Gymno-sporangium and Hemipuccinia, bears asexually produced spores, teleutospores; in Puccinia Graminis the formation of teleutospores is preceded by that of somewhat different spores, the uredospores ; in any case the teleutospore gives rise, on germination, to a second asexual generation, the promycelium, which bears sporidia ; from these the sexual mcidium-bearing generation is developed.
The rule that, in the alternation of generations, the alter-nate generations are developed from spores produced either sexually or asexually is not, however, without exceptions, for in some instances the one generation may spring vege-tatively from the other without the intervention of a. spore. This is brought about by the suppression either of the sexually produced spore or of the asexually produced spore; the former is an instance of apogamy, the latter of apospory.
Thus in the apogamous Ferns mentioned above, the asexual generation (sporophore) is developed as a bud upon the sextal generation (oophore) ; and in Botrydium the gametangium which is the representative of the sexual generation may, instead of producing gametes, produce zoospores, in which case the new asexual individual is not developed from a sexually produced zygospore, but from an asexually produced zoospore. Similarly, in the apo-sporous Ferns and Mosses and in the Characem the oophore is developed as a bud from the sporophore.
The alternation of generations may be also interfered with by a combination of apogamy and apospory. This is the case, namely, 1-vhen one generation gives rise to its like by vegetative budding, sporophore to sporophore, oophore to oophore. For instance, when, as in the Phanerogams mentioned above (Ccelebogyne, Funkia, Nothoscordum, Citrus), embryos are produced vegetatively from the tissue of the nucellus, that is, sporophore from sporophore, the typically intervening formation of spores, first, by the asexual method and, secondly, by the sexual method is suppressed. This is necessarily always the case among Phanerogams when one plant is produced vegetatively from another. A striking instance of the same thing has been observed by Goebel in some species of Isoetes, in which an Isoetes plant was produced on the leaf in place of a sporangium. Similarly when oophore springs vegetatively from oophore, the typically intervening formation, first, of the sex-ually produced spore and, secondly, of the asexu-ally produced spore, is suppressed. This occurs when, as mentioned above, a Moss-plant gives rise by budding. or by means of gemnim to another Aioss-plant, and when a Fern-prothallium gives rise to another by means of gernnam.