Plants are organisms belonging to the kingdom Plantae (see below), characterized by their ability to manufacture carbohydrates by photosynthesis. With more than 250,000 species, they are second in size only to the arthropods. The plants first appeared in the Ordovician, but did not begin to resemble modern plants until the Late Silurian. By the close of the Devonian, about 360 million years ago, there were a wide variety of shapes and sizes of plants around, including tiny creeping plants and tall forest trees.
Green plants are unique in being able to synthesize their own organic molecules from carbon dioxide and water using light energy. Mineral nutrients are absorbed from the environment. Plants are the primary source of food for all other living organisms.
The possession of chlorophyll, the green photosynthetic pigment, is probably the most important distinction between plants and animals, but there are several other differences. Plants are stationary, have no nervous system, and the cell wall contains large amounts of cellulose. But there are exceptions. Some organisms which can be classified as plants, such as algae and bacteria, can move about, and others, such as fungi, bacteria, and some parasites, do not contain chlorophyll and cannot synthesize their own organic molecules, but absorb them from their environment. Some insectivorous plants obtain their food by trapping insects.
Although the more primitive plants vary considerably in their overall structure, the higher planets (gymnosperms and angiosperms) are much the same in their basic anatomy and morphology. In a typical angiosperm, four main regions can be recognized: root, stem, leaf, ad flower. Each region has one or more basic functions.
When examined under the microscope, a piece of plant tissue can be seen to consist of tiny cells, generally packed tightly together. The cells are not all alike and each one is adapted to do a certain job. All are derived, however, from a basic pattern. This basic plant cell tends to be rectangular and has a tough wall of cellulose which gives it its shape, but the living boundary of the cell is the delicate cell membrane just inside the wall. Inside the membrane is the cytoplasm, which contains the nucleus, the chloroplasts, and many other microscopic structures. In the center of the cytoplasm is a large sap-filled vacuole, which maintains the cell's shape and plays an important part in the working of the whole plant.
Both sexual and asexual reproduction are widespread throughout the plant kingdom. Many plants are capable of both forms and in some cases the life cycle of the plant may involve the two different forms (see alternation of generations).
The plant kingdom
The plant and animal kingdoms together embrace all living things except viruses, and only overlap in the most primitive organisms. The plant kingdom is extremely diverse (over 400,000 species are now known), and they are found in almost every conceivable habitat. They range in size from microscopic bacteria to 100 meters (300ft) sequoias.
The plant kingdom can be arranged into an orderly hierarchical pattern of classification (see taxonomy) containing divisions, classes, orders, families, genera, and species. Indeed, several systems have been devised to do this. In the classical Eichler system there are four divisions: the thallophyta, including bacteria, slime mold, algae, and fungi; the bryophyta, including liverworts, hornworts, and mosses; the pteridophyta, including ferns, club mosses, and horsetails; and the spermatophyta, including gymnosperms and angiosperms, the latter being divided into dicotyledons and monocotyledons. However, this system was subsequently replaced by a more natural arrangement of 11 divisions: Schizophyta, bacteria and cyanobacteria; Euglenophyta, euglenoids; Chlorophyta, green algae; Xanthophyta, yellow-green algae; Chrysophyta , golden algae and diatoms; Phaephyta, brown algae; Rhodophyta, red algae; Pyrrophyta, dinoflagellate and cryptomonads; Mycota, slime molds and fungi; Bryophyta, liverworts and mosses; and Tracheophyta, the vascular plants, including horsetails, ferns, gymnosperms, and angiosperms. Under this system some authorities break the plant kingdom into three kingdoms: the Monera, including the division Schizophyta; the Metaphyta, including the Bryophyta and Trachaephyta; and the Protista, which includes all the other divisions.