Classification Of Plants
All species are classified hierarchically. Related species are grouped into a genus; related genera into a family; related families into an order; related orders into a class; related classes into a phylum; and related phyla into a kingdom. Below, the most significant characteristics of the nine phyla of the kingdom Plantae are briefly considered.
Bryophyta is a phylum with three classes, the largest of which is the mosses, with about 15,000 species. The gametophyte phase is dominant, and in mosses this is the familiar, small, green, leafy plant. Bryophytes do not have true leaves, stems, or roots, and they lack a vascular system for transporting food and water. They reproduce by making spores, and are mostly found in bogs or moist woodlands, so their sperm can swim through water to reach the eggs. Mosses are particularly prominent in the northern boreal forest and arctic and alpine tundra.
The Lycopodophyta is a phylum with about 1,000 species. The sporophyte phase is dominant, and is the familiar, low-growing, green plant in many species which superficially resembles the branch of a pine. Their leaves are tiny structures, termed microphylls, and are arranged in whorls on the stem. The stems of lycopods and all subsequent phyla have vascular tissues for efficient transport of food and water. Like bryophytes, they reproduce by making spores, and are mostly found in wet areas so their sperm can swim to reach the eggs. Lycopods are most abundant in the tropics, although numerous species of Lycopodium (ground pine) grow in woodlands in the temperate zone.
The Sphenophyta has a single genus, Equisetum, with about 10 species. Equisetum is commonly called horsetail, because the dominant sporophyte phase of these plants superficially resembles a horse's tail. It is an erect stem, with whorls of microphylls, and a spore-producing, cone-like structure, termed a strobilus, on top. Horsetails are mostly found in moist woodlands of the temperate zone, since their sperm must swim to reach the eggs.
The Filicinophyta has about 11,000 species, which are known commonly as ferns. The sporophyte phase is dominant, and is the more familiar form of ferns that is commonly seen in temperate-zone woodlands. Like the leaves of all subsequent phyla, those of ferns have a complex system of branched veins, and are referred to as megaphylls. Ferns reproduce by making spores, and they are mostly restricted to moist environments so their sperm can swim to reach the eggs. Most species occur in tropical and subtropical ecosystems.
The Cycadophyta has about 200 species, which are known commonly as cycads. Like all subsequent phyla, cycads are seed-producing plants. They are considered gymnosperms, because they bear their seeds naked on specialized leaves called sporophylls. The sporophyte phase is dominant, and appears rather like a shrublike palm in many species, although cycads are only distantly related to palms. Cycads have flagellated sperm which swim to fertilize the eggs, a characteristic of evolutionarily primitive, free-sporing plants (all phyla above), but not of other seed plants (except for Ginkgo, see below). Cycads grow in tropical and subtropical regions of the world.
The Ginkgophyta consists of a single species, Ginkgo biloba, a gymnosperm which bears its seeds in green, fruit-like structures. The sporophyte phase of Ginkgo is dominant, and is a tree with fan-shaped leaves that arise from spurs on the branches. Like the cycads, Ginkgo has flagellated sperm that swim to fertilize the eggs. Ginkgo only exists in cultivation, and is widely planted as an ornamental tree throughout the United States and other temperate countries.
The Coniferophyta has about 600 species, and includes familiar evergreen trees such as pines, spruces, and firs. The conifers are the best known and most abundant of the gymnosperms. The sporophyte phase is dominant, and is the familiar cone-bearing tree. Male reproductive structures produce pollen grains, or male gametophytes, which travel by wind to the female reproductive structures. The pollen fertilizes the ovules to produce seeds, which then develop within characteristic cones. Conifers grow throughout the world, and are dominant trees in many northern forests. Many conifers are used for lumber, paper, and other important products.
The Gnetophyta is a phylum of unusual gymnosperms, with about 70 species in three genera, Gnetum, Ephedra, and Welwitschia. These three genera differ significantly from one another in their vegetative and reproductive structures, although all are semi-desert plants. The mode of fertilization of species in the Ephedra genus resembles that of the Angiospermophyta (flowering plants), and many botanists consider them to be close relatives.
The Angiospermophyta is the largest and most important plant phylum, with at least 300,000 species. All species reproduce by making flowers, which develop into fruits with seeds upon fertilization. The flower originated about 130 million years ago, as a structure adapted to protect the ovules (immature seeds), which are born naked and unprotected in the more primitive gymnosperms. The highly specialized characteristics of many flowers evolved to facilitate pollination. There are two natural groups of angiosperms, the monocots, whose seeds have one cotyledon (or seed-leaf), and the dicots, whose seeds have two cotyledons. Nearly all of the plant
foods of humans and many drugs and other economically important products come from angiosperms.
A recent scientific effort has created new theories about the classification of plants. Many genetic experiments were performed by plant biologists around the world in an effort to answer questions of the evolution of plants as a single large group of organisms. Some startling, and controversial results were attained just before the turn of the new century. In 1999, the group of scientists concluded that the kingdom Plantae should, in fact, be split into at least three separate kingdoms because the group is so highly diverse and the genetic evidence gathered indicated sufficient divergence among members. Also, the studies uncovered that the three proposed kingdoms each formed from a single plant-like ancestor that colonized land, not directly from the sea as was previously thought, but from fresh water. These ideas have yet to be accepted by the majority of biologists, and remain a matter of debate.