Of the more advanced living organisms on earth, the plants evolved before the animals. The oldest plant fossils date from the Precam-brian era, over 3 billion years ago. It was not until the Cambrian period almost 600 million years ago, however, that animals evolved, by which time plants had prepared the conditions in which animal life could exist. The green plants manufactured sugar by photosynthesis and in doing so released oxygen, which revolutionized the atmosphere of the planet. Only then could the air be utilized by animals.
Since the first appearance on earth of plant life, hundreds of thousands of plant species have populated the planet, many of which are now extinct. Today about 400,000 species of plants are known, although the true number is probably considerably more, because new species are constantly being discovered and identified. Traditionally, the plants are divided into the lower, or nonflowering plants, and those that produce flowers and seeds.
What is a plant?
Plant cell structure usually distinguishes these organisms from animals. The cell has a rigid, cellulose wall, rather than a flexible membrane. Large, permanent vacuoles occur in the center of the cytoplasm, whereas animal cells have small temporary ones, or none at all.
Plant cells have chloroplasts that contain chlorophyll; animal cells do not.
A plant is generally considered to be a living organism that manufactures its own food (it is autotrophic). There are some, however, that are parasitic, and others that are saprophytes, processing food outside their bodies and then absorbing it. Plants are usually immobile, functioning on the spot.
Even so, there is some overlap between lower animals and simple plants, and the distinction between them is not always clear. Many unicellular and some multicellular algae move around like animals, but are autotrophic; some surround and ingest their food (they are phagotrophic) like protozoa, but do not have a true cell wall, only a membrane; slime molds are also phagotrophic but have a cell membrane during their feeding stage only and, unlike protozoa, reproduce by spores (they are sporophytic).
The most primitive lower plants have no root structures, conducting or supporting tissues. Vascular plants, in contrast, have a system of vessels that transport water and nutrients. These higher plants are differentiated into stems, leaves, roots and flowers; they also have supporting tissues. Most lower plants are spore-producing, or sporophytic, whereas the higher plants are seed-producing, or sperma-tophytic.
The simplest organisms are the bacteria and cyanobacteria, which are called prokaryotes because they have no distinct nucleus in their cells. All other plants are called eukaryotes— that is, each cell has a true nucleus and more complex organelles. Bacteria are single-celled but contain no chlorophyll and, rather than surviving by photosynthesis, use a variety of biochemical means to obtain energy. Some manufacture their energy supplies from chemicals in their immediate environment whereas others rely on finding it ready-made. These bacteria include the disease-forming types that inhabit the bodies of humans and other animals. They reproduce simply by splitting in two (binary fission) and, occasionally, by spores. Bacteria comprise about 4,000 species.
The cyanobacteria (blue-green algae) are grouped with the bacteria because they share some structural features: for example, their DNA is not arranged into chromosomes; and they do not have a nuclear membrane. Some of the other algae are more closely related to the higher plants in their cell structure. Many, such as Euglena, consist of single cells capable of independent existence. Others are composed of chains (filaments) of cells, and the large seaweeds are made up of thousands of cells. They also vary widely in their pigmentation, cell wall composition, and the kind of foodstuff they store. Algal reproduction may be asexual—by binary fission—or sexual, by gametes. There are approximately 21,000 species of algae.
In common with the bacteria, the fungi do not contain chlorophyll and, therefore, do not photosynthesize. Instead, they live either as parasites on other plants or animals, obtaining food ready-processed or as saprophytes. Like the bacteria, they function as decomposers, releasing nutrients that support higher organisms. Fungi include molds, yeasts, rusts, smuts, mushrooms, and toadstools. This group comprises more than 100,000 species. Most reproduce sexually and asexually, the typical mushroom-shaped fruiting body producing spores by sexual means.
The lichens are compound organisms of fungi and algae, which have a symbiotic relationship. This successful group is autotrophic and survives in almost every habitat; it contains about 18,000 species.
Mosses and liverworts (the bryophytes) are represented by more or less the same number of species as lichens are. They photosynthesize but are nevertheless regarded as primitive land-dwelling plants because they do not have roots or a truly differentiated vascular system. These green plants typically have an alternation of sexual and asexual generations (ga-metophyte and sporophyte respectively).
The ferns, club mosses, and horsetails—the pteridophytes—do have a vascular system. Like the mosses, these plants also display alternation of generations, but the mature plant (the sporophyte) is much more complex than that of the bryophytes, with differentiated growing and conducting tissues. Therefore, they can grow much larger in size. There are about 12,000 known species of pteridophyte, which are distributed worldwide; this is, however, a tiny fraction of the number that dominated the earth about 400 million years ago.
The higher plants are distinguished from the lower ones by the development of the seed and with it the restriction of the gametophyte (sexual) generation to a fusion of cells within the sporophyte. This development reduced the risks of the reproductive process and (except in primitive forms such as cycads and ginkgos) lessened the need for water as an essential medium for the completion of fertilization.
About 250,000 species of seed-producing plants are recognized today. Of these some 500 species are gymnosperms (conifers and cycads). Typically, the seeds of gymnosperms are naked, compared with those of angio-sperms, whose seeds are enclosed in a tissue coating called an ovary. The angiosperms, or flowering plants, now dominate the earth’s vegetation, having adapted to survive in almost every habitat.