Plants (of the kingdom Plantae) are multicellular, eukaryotic organisms that develop from an embryo and that have cell walls and chloroplasts. Plants are distinguished from algae (from which they are descended) by a higher degree of multicellular complexity and from fungi by the ability to photosynthesize (those few plants that have lost this ability evolved from others that could).
Characteristics of Plants
Almost all plants live on land and have adapted to the conditions on land through the development of a waxy cuticle to prevent drying out, structures to absorb and transport water throughout their bodies (the bryophytes are an exception), and rigid internal support to remain erect without the buoyancy available in water. This rigidity is provided in large part by the cell wall, which is composed of cellulose , a complex carbohydrate , and lignin , a phenolic compound that stiffens the cellulose fibers.
The plant life cycle has two distinct multicellular phases: a haploid phase (in which chromosomes are present only as single copies) and a diploid phase (in which chromosomes are present in pairs). The haploid organism produces gametes that fuse to form an embryo, which develops into the diploid organism. The diploid organism produces haploid spores that germinate to form the haploid organism. This "alternation of generations" is found only in plants and some algae.
Almost all plants photosynthesize, using the sun's energy to power the production of sugar from carbon dioxide and water. Photosynthesis occurs in chloroplasts, membrane-bound organelles that contain the green pigment chlorophyll. Chloroplasts are descended from free-living photosynthetic bacteria that became symbiotic partners of ancient single-celled plant ancestors. Evidence of the chloroplast's bacterial origin is found in the presence of deoxyribonucleic acid (DNA) within it, as well as its size and structure.
The photosynthetic production of sugars by plants is the basis for all terrestrial food chains. Photosynthesis also produces oxygen, needed by animals, fungi, and other organisms (including plants themselves) to release the stored energy in those sugars.
Plants are classified into twelve phyla (sometimes called divisions) in two major groups. The bryophytes are the most primitive group, lacking vascular tissues for the transport of water. There are three phyla of bryophytes—the mosses, liverworts, and hornworts—that together comprise about 24,000 species. In contrast, plants in the second group, the tracheophytes, have well-developed vascular systems. The tracheophytes contain nine phyla and are divided into two groups: those without seeds and those with them. Ferns, which reproduce without seeds, contain approximately 13,000 species. Three other phyla of seedless vascular plants (Psilophyta, Lycopodophyta, and Equisetophyta) together include just over 1,000 species.
Seeds are structures that contain an embryo and food reserves wrapped in a protective seed coat. In the gymnosperms , the seed develops on structures exposed to the environment. Gymnosperms include Ginkophyta, which contains only one species, Ginkgo biloba ; Cycadophyta (220 species); Gnetophyta (68 species); and Coniferophyta (588 species). Conifers bear seeds in cones and include many familiar needle-bearing evergreens, such as pine, spruce, and fir. Anthophyta, or angiosperms, enclose their seeds within ovaries. The angiosperms are the flowering plants and are the most diverse of all plant phyla, with about 235,000 species.
Raven, Peter, Ray F. Evert, and Susan E. Eichhorn. Biology of Plants, 6th ed. New York: W. H. Freeman and Company, 1999.