Plants belong to the Plantae kingdom. Biologists estimate that there are up to 350,000 species making up this kingdom. In general, there are two types of land-growing plants—vascular and nonvascular. Vascular plants have specially developed organs similar to veins that move liquids through their systems. This category includes the trees, shrubs, flowers, and grasses. Nonvascular plants are mosses, liverworts, and hornworts. The vast majority of plant species on Earth are vascular plants that reproduce through their flowers.
In science, plants are more often identified by their scientific names than are animals. Plant species are so abundant and diverse that many plants have multiple common names. On the other hand, there are plants that have no common names because they are rare or geographically remote. To avoid confusion, this chapter will include the scientific name for any specific common name given.
Many factors contribute to the endangerment of plant species. Numerous species are the victims of habitat loss due to land and agricultural development. Others have declined due to pollution or habitat damage, or as a result of competition with invasive species. Still others have succumbed to introduced or unknown plant diseases. Finally, collectors or dealers often illegally seek rare, showy, or unusual plants, and have depleted populations through over-collection.
The preservation of plant species is important for many reasons. Not only are plants of aesthetic value, they are crucial components of every ecosystem on earth. Plants also serve several functions directly beneficial to humans. First, they provide genetic variation that is used in the breeding of new crop varieties—native plants provide genes that allow for adaptation to local environments, as well as resistance to pests, disease, or drought. In addition, plants are the source of numerous human medicines.