In the classification above, only the major divisions and classes of living plants are listed, and a number of entirely extinct divisions are omitted. The classification outlined is somewhat conservative but is one that best conforms to available data and has gained wide acceptance.
Biological classifications were initially mechanical or “artificial”; that is to say, they had no basis in evolution. This was followed by a period of “natural system” construction, whereby plants were grouped together on the basis of their overall similarities or differences, using as many characteristics as possible. Contemporary systems of biological classification are phylogenetic, which means that various plants are arranged together because they are thought to be related by descent from a common ancestor. As additional molecular evidence has become available, classifications have changed to accommodate the new information.
At the turn of the 19th century, the plant kingdom was frequently divided into two major groups, the cryptogamia (algae, fungi, bryophytes, and ferns) and the phanerogamia (gymnosperms and angiosperms). Subsequently, it was common practice among systematic botanists to group all vascular plants together under a single division, Tracheophyta.
More modern taxonomies, such as those of the Angiosperm Phylogeny Group (APG), do not formally recognize groupings at the division level but use more informal groups known as clades, a view that interprets the individual major groups to be less closely related to one another than was previously believed. Difficult and complex questions still exist in the definition and circumscription of certain groups. The phylogenetic relationships, if any, of the bryophytic plants with primitive vascular plants remain unclear.
KINGDOM PLANTAEMulticellular photosynthetic eukaryotic organisms containing chlorophylls a and b and carotenoids and forming true starch; chlorophyll contained in stacks of membranes (grana) within chloroplasts; cellulosic cell walls; aerial tissues covered with a waxy cuticle and provided with openings (stomata) in the epidermis bordered by 2 guard cells and serving in gas exchange; alternation of generations with multicellular gametangia and sporangia and possessing a multicellular embryo stage; motile sperm, when present, with whiplash flagellum; sperm with a microtubular cytoskeleton; cell division associated with the dispersal of the nuclear membrane and the formation of a cell plate across the mitotic spindle (phragmoplast).
Division Bryophyta (mosses)Small, mostly nonvascular, archegoniate plants with a dominant photosynthetic free-living gametophyte; sporophyte has little or no chlorophyll and is dependent on gametophyte; biflagellate sperm. Gametophytes “leafy” and radially symmetrical, with leaflike structures arising spirally from stemlike axis; many chloroplasts per cell; gametophyte with multicellular rhizoids; sporophytes with complex spore-containing capsule with peristome and operculum, columella, and stomata present but elaters absent; between 10,000 and 14,000 species; representative genera include Polytrichum, Mnium, Funaria, and Sphagnum.
Division Anthocerotophyta (hornworts)Gametophyte thalloid, with a single large chloroplast per cell, mucilage cavities present; sporophytes persistent, erect (hornlike), photosynthetic, arising from upper surface of gametophyte, possessing stomata, columella, basal meristem, and pseudoelaters opening by 2 valves but lacking an operculum; representative genus, Anthoceros.
Division Marchantiophyta (liverworts)Gametophytes either “leafy” or dorsiventrally flattened (strap-shaped) and thalloid; leafy forms with leaves in 3 rows, 2 lateral and 1 below; leaves usually notched or lobed, with thickened midrib lacking; gametophytes with unicellular rhizoids; many chloroplasts per cell; sporophytes ephemeral, surrounded by gametophytic tissue, lacking opercula, peristome, stomata, and columella; elaters mixed with spores in capsule; capsule opening into 4 or more valves; between 6,000 and 9,000 species; representative genera include Porella, Frullania, Marchantia, Conocephalum, and Riccia.
Division Lycophyta (club mosses, spike mosses, quillworts)Vascular plants; sporophyte differentiated into stem, leaf, and root; leaves spirally arranged on stem, of the microphyll type; sporangia borne on upper (adaxial) surface of leaves (sporophylls), 1 sporangium per sporophyll; vascular tissue basically forming a central core in stem (protostelic); homosporous or heterosporous; ligulate (having basal leaflike protuberances) or eligulate; strobili (cones) present or lacking; all living genera with primary growth only; gametophytes subterranean or surface-dwelling; motile sperm; about 5 living genera and between 600 and 1,000 species; representative genera include Lycopodium, Selaginella, and Isoetes.
Class Polypodiopsida (leptosporangiate ferns)Vascular plants; sporophyte differentiated into stem (rhizome), roots, and leaves (fronds); leaves entire or, more often, divided; arrangement of vascular tissue in stem variable, primary growth only; sporangia usually clustered into sori, often located on the under (abaxial) surface of sporophylls; mostly homosporous; gametophytes (prothallia) either subterranean and nongreen or, more commonly, surface-dwelling and photosynthetic; sperm motile; between 9,000 and 12,000 species; representative genera include Pteridium, Polypodium, Polystichum, Adiantum, and Cyathea.
Class Equisetopsida (horsetails, scouring rushes)Vascular plants; sporophyte differentiated into stem, leaf, and root; stems ribbed and jointed, monopodial; minute leaves whorled at the nodes; vascular tissue organized into bundles; sole living genus with primary growth only; sporangia borne on specialized stalks (sporangiophores) in strobili; homosporous; gametophytes photosynthetic, surface-dwelling; motile sperm; 1 genus, Equisetum.
Class Marattiopsida giant fernsVascular plants; mostly massive, fleshy ferns; leaves pinnately divided, pulvinate (enlarged or swollen at attachment point of leaflets) in extant genera, and with well-developed fleshy stipules (appendages at leaf base); sporangia eusporangiate, in sori, or more or less coalescent in synangia (clusters); homosporous; representative genera include Marattia, Angiopteris, and Danaea.
Class Psilotopsida (whisk ferns)Vascular plants; sporophyte lacking roots and often leaves; stems with small enations, dichotomously branched; vascular tissue forming a central core in stem (protostelic); sporangia fused into synangiate structure, apparently terminal on short stem; homosporous; gametophytes subterranean, with motile sperm; representative genus, Psilotum.
Division Cycadophyta (cycads)Palmlike gymnospermous plants with typically short, thick, unbranched aerial trunks, sometimes subterranean, and large, divided leaves; leaves usually thick and leathery; cones present, often large and terminal on the stem; pollen (male) and seed (female) cones borne on separate plants; gametophytes reduced, not free-living; sperm motile (flagellated) but taken to vicinity of egg by a pollen tube; 10 genera and about 120 species; representative genera include Zamia and Cycas.
Division Ginkgophyta (ginkgoes)Gymnospermous plants; tall much-branched tree with well-developed cylinder of wood; resin ducts present; xylem with tracheids only; stem differentiated into long shoots and short spur shoots; simple fan-shaped leaves with open dichotomous venation terminate short shoots; leaves deciduous; sexes on separate trees; distinct cones lacking; gametophytes reduced, not free-living; sperm motile (flagellated); mature seeds with fleshy, foul-smelling outer region; 1 living species, Ginkgo biloba.
Division Gnetophyta (gnetophytes)Diverse and unusual group of 3 gymnospermous genera; growth habits of all 3 are atypical among gymnosperms in possessing vessel elements in the xylem and reproductive structures that are somewhat flowerlike; gametophytes reduced as in other gymnosperms; sperm nonmotile; extant genera Gnetum, Ephedra, and Welwitschia.
Division Pinophyta (conifers)Gymnospermous plants; mostly trees with abundant xylem composed of tracheids only; resin ducts present; leaves simple, needlelike, scalelike, with a single vein or, less commonly, strap-shaped with multiple veins; reproduction by well-defined cones; seeds exposed on ovuliferous scales; gametophyte generation reduced, microscopic, not free-living; sperm nonmotile, transported to egg by pollen tube; approximately 68 genera and some 630 species; representative genera include Pinus, Abies, Sequoia, Taxodium, Juniperus, Podocarpus,Cupressus, Taxus, and Agathis.
Angiosperms, unranked (flowering plants)Vascular plants, xylem typically with vessel elements; reproduction by flowers; ovules or young seeds enclosed by female reproductive structure (carpel); gametophyte generation extremely reduced, consisting of only a few cells; archegonia and antheridia lacking; nonmotile sperm transported to egg by pollen tube; pollen transported to specialized receptive surface (stigma) on carpel; double fertilization, one sperm uniting with the egg to form a zygote, another fusing with the polar nuclei to form the primary endosperm nucleus; nutritive tissue of seed triploid endosperm; seeds enclosed by mature ovary that ripens into a fruit; more than 300,000 species worldwide, more than 300 families.
A series of changes in the reproductive biology among some heterosporous plants during the late Devonian overcame this environmental restriction and allowed them to colonize a much wider range of habitats. These changes also led to the evolution of seed plants. In seed plants, the megaspore is retained in the megasporangium and the microspore is taken to a pollen chamber at the tip of this organ. The megasporangium is then sealed, and gametophyte development and fertilization occur within a protected environment. When such a megasporangium is enclosed in a seed coat, the structure with its enclosed embryo is called a seed. Seeds of the earliest such plants were exposed to the external environment; those “naked seed” plants are referred to as gymnosperms.
From the late Devonian through the base of thelate Cretaceous Period (about 382.7 million to 66 million years ago), gymnosperms underwent dramatic evolutionary radiations and became the dominant group of vascular plants in most habitats. Extant gymnosperms include conifers, cycads, and Ginkgo biloba, but those represent only a small fraction of the gymnosperms that inhabited Earth during the Mesozoic Era (251.9 million to 66 million years ago). Among the Mesozoic forms were species with a wide variety of mechanisms for effecting pollination, protecting the seeds, dispersing the seeds, and increasing the natural selection of the most successful varieties.
Primitive forms of the flowering plants (angiosperms) arose from among this diverse array of complex gymnosperms. From their earliest diversification in the Cretaceous Period (145 million to 66 million years ago), angiosperms rapidly came to dominate the land flora. Today there are more than 300,000 species of flowering plants, which account for more than 90 percent of the diversity of vascular plants. Among the many structures that contribute to this success are flowers, fruits, complex vein patterns in the leaves, and highly specialized cells of the conducting system.
Just as an ecological succession of plant forms can transform bare ground into a complex plant community, so an evolutionary succession can be thought of as having transformed bare continents into rich terrestrial biotas. In ecological succession, herbaceous plants colonize bare earth and modify the environment for the development of more complete ground cover. This further modifies the environment for the successive establishment of larger herbs, perennial shrubs, fast-growing trees, and, finally, slower-growing trees, vines, and epiphytes (plants that grow on other plants rather than in soil).
As discussed above, primitive land plants of the late Silurian and early Devonian periods were primarily small herbs that inhabited the moist lowlands near oceans, lakes, and streams. The first of these grew on bare ground because there would have been little or no organic soil, because plants had yet to produce the organic matter that gave rise to organic soils. They may be thought of as the evolutionary equivalent of primary colonizing “weeds,” and their establishment on land was responsible for the first of three dramatic increases in the diversity of land plants.
After the initial production of organic soils by these evolutionary colonizers, communities of larger herbs and shrubs were able to develop, and they became common in the middle Devonian. In the late Devonian, these communities were themselves succeeded by communities dominated by heterosporous tree-sized plants. During the early Carboniferous Period, nonseed plants continued to dominate many wetland habitats, whereas communities dominated by gymnosperm trees colonized drier habitats than had been previously available to the heterosporous forms.
Communities dominated by trees were the first to provide suitable habitats for vines and for epiphytes. Such communities also provided habitats for herbivorous animals. As a result, biological communities of considerable complexity had evolved by the late Carboniferous Period (about 323.2 million to 298.9 million years ago), giving rise to a second dramatic increase in the diversity of land plants.
With the rapid diversification of the gymnosperms and an increasing complexity of interactions between plants and animals, there was a significant evolutionary turnover among land plants during the Permian, Triassic, and Jurassic periods (298.9 million to 145 million years ago). New groups of gymnosperms replaced some of the primitive gymnosperms as well as the most prominent nonseed plants from earlier periods. Among the most successful gymnosperms of those periods were conifers, cycads, Ginkgo, and several major groups with no extant representatives.
Flowering plants probably also originated during this time, but they did not become a significant part of the fossil flora until the middle of the Cretaceous Period. The fossil evidence provides a clear picture of the rapid diversification and spectacular rise to dominance of the angiosperms during the late Cretaceous Period. This was accompanied by a dramatic increase in the complexity of plant community structure, produced at least in part by an expanding array of interactions between plants and animals. Improved pollination and seed dispersal were among the benefits of such interactions to plants.
That animals equally benefited is evidenced by the coevolution of several groups of animals (particularly insects) and angiosperms during the late Cretaceous and Paleogene periods (about 100.5 million to 23 million years ago). Such interactions contributed significantly to a third rapid increase in global plant diversity and helped angiosperms achieve the overwhelming dominance of the land flora characteristic of modern vegetation.
Botanists now believe that plants evolved from the algae; the development of the plant kingdom may have resulted from evolutionary changes that occurred when photosynthetic multicellular organisms invaded the continents. The earliest fossil evidence for land plants consists of isolated spores, tracheid-like tubes, and sheets of cells found in Ordovician rocks.
The abundance and diversity of these fossils increase into the Silurian Period (about 443.8 million to 419.2 million years ago), where the first macroscopic (megafossil) evidence for land plants has been found. These megafossils consist of slender forking axes that are only a few centimetres long. Some of the axes terminate in sporangia that bear trilete spores (i.e., spores that divide meiotically to form a tetrad). Because a trilete mark indicates that the spores are the product of meiosis, the fertile axes may be interpreted as the sporophyte phase of the life cycle.
Fossils of this type could represent either vascular plants or bryophytes. Another possibility is that they are neither but include ancestors of vascular plants, bryophytes, or both. The earliest fossils also include at least one or more additional plant groups that became extinct early in the colonization of the land and therefore have no living descendants.
By the early Devonian Period (about 419.2 million to 393.3 million years ago), some of the fossils that consist of forking axes with terminal sporangia also produced a central strand of tracheids, the specialized water-conducting cells of the xylem. Tracheids are a diagnostic feature of vascular plants and are the basis for the division name, Tracheophyta.
The simplest and presumably most primitive vascular plants from the late Silurian and early Devonian periods (about 419.2 million to 393.3 million years ago) were the Rhyniopsida. They included plants such as Cooksonia and Rhynia, which were herbaceous colonizers of moist habitats. Most were less than 30 cm (12 inches) tall. The plant body was not differentiated into stems, leaves, and roots; rather, the forking aboveground axes bore terminal sporangia and produced stomata, which demonstrate that the plants were green and photosynthetic.
Surface or underground axes served to root the plant and were anchored by rhizoids. Because such plants produced only one type of spore, they were nonseed plants with a homosporous life cycle and free-living gametophytes. A small number of such gametophytes have been described from Devonian deposits.
Some plants of the early Devonian had multicellular emergences of tissue along their aboveground axes, which are thought to have increased the light-capturing surface of the photosynthetic tissue. Such emergences (enations) gave rise to the leaves (microphylls) of the Lycopsida, thus producing an aboveground shoot system that consisted of branching stems with leaves. Underground axes that lacked leaves would have become the roots. Lycophytes were the first plants with well-differentiated shoot systems, and they are the most ancient groups of vascular plants with living representatives.
The leaves of several other plant groups were derived from modifications of the forking axes. There was a variety in structure among Devonian plants. The axes of some plants forked equally, whereas other plants—e.g., Drepanophycus—were more specialized, comprising a large centrally located axis as well as smaller axes borne laterally. In plants where the lateral systems branched in only one plane, the side branches were flat like leaves. Filling in (webbing) of the spaces between forks of the laterals with photosynthetic tissue produced leaves called megaphylls.
There is evidence for the evolution of megaphylls in several plant groups of the late Devonian Period (about 382.7 million to 358.9 million years ago) and early Carboniferous Period (about 358.9 million to 323.2 million years ago). Although most of these groups have no living representatives, by the Carboniferous Period they had given rise to homosporous ferns, sphenopsids (horsetails), and seed plants (gymnosperms).
As Devonian plants with microphylls and those with specialized branching systems diversified, many grew to the size of shrubs. By the middle Devonian Period (about 393.3 million to 382.7 million years ago), there were shrub-size representatives of several lineages, but a further increase in the size of herbaceous plants was restricted by the limited diameter that aboveground stems and rooting systems could achieve. The development of lateral (secondary) growth overcame this size restriction. The ability to produce secondary growth evolved independently in several groups.
In the lycophytes, much of this secondary growth occurred in cortical tissues; in the ancestors of seed plants and several other lineages, however, the production of wood accounted for most of the growth in stem diameter. About the same time, downward-growing central rooting systems evolved independently in lycophytes and other plant groups. As a result, there were forests with a canopy of giant lycophyte and gymnosperm trees near the beginning of the Carboniferous Period.
As plants developed more complex growth forms, they also underwent systematic diversification and evolved more specialized modes of sexual reproduction. The most primitive vascular plants had a homosporous life cycle, in which reproduction and dispersal involved a single type of spore. Extant homosporous plants include most ferns and many lycophytes.
The homosporous life cycle is an effective means for long-distance dispersal of species. Although it permits the fertilization of an egg by a sperm from the same gametophyte plant, genetic recombination, considered important for more rapid evolution, is not possible. Moreover, because the gametophytes of homosporous plants are exposed to the environment for an extended period of time, mortality is relatively high.
By the middle Devonian Period, the heterosporous life cycle had evolved independently in several groups, including lycophytes and the ancestors of seed plants. In heterosporous plants, there are two sizes of spores; the smaller (a microspore) produces a male gametophyte, and the larger (a megaspore) produces a female gametophyte.
The incidence of genetic recombination is increased by this obligate cross-fertilization. Both types of gametophytes develop quickly within the protective spore wall. Compared with homosporous plants, reproduction is more rapid and mortality is reduced in heterosporous plants. By the end of the Devonian, heterosporous plants had become the dominant species in most wetland environments; however, the need for an abundant source of water from the environment to allow fertilization prevented the heterosporous plants from establishing communities in drier habitats.
The evolutionary history of plants is recorded in fossils preserved in lowland or marine sediments. Some fossils preserve the external form of plant parts; others show cellular features; and still others consist of microfossils such as pollen and spores. In rare instances, fossils may even display the ultrastructural or chemical features of the plants they represent. The fossil record reveals a pattern of accelerating rates of evolution coupled with increasing diversity and complexity of biological communities that began with the invasion of land and continued with the progressive colonization of the continents.
At present, fossil evidence of land plants dates to the Ordovician Period (about 485.4 million to 443.8 million years ago) of the Paleozoic Era. However, research using “molecular clock” methodology, which uses genetics to estimate how long species have been separated from a common ancestor, suggests that plants started to colonize terrestrial environments around 500 million years ago, about the middle of the Cambrian Period.
By far the most diverse and conspicuous living members of the plant kingdom are vascular plants (tracheophytes), in which the sporophyte phase of the life history is dominant. (See above Life histories.) Fossil remains of vascular plants provide evidence for evolutionary changes in the structure of the plant body (sporophyte and gametophyte), in the variety of plant forms, in the complexity of the life history, in the tolerance for ecological conditions, and in systematic diversity.
Nonvascular plants, or bryophytes (mosses, liverworts, and hornworts), are much smaller and less diverse than vascular plants. The first evidence for liverworts occurs in rocks laid down between 473 million and 471 million years ago, during the Ordovician Period, whereas the earliest moss fossils are from the Permian Period (298.9 million to 251.9 million years ago). In contrast to tracheophytes, most fossil bryophytes are relatively similar to living forms. Understanding of the evolution of nonvascular plants is, therefore, less complete than for tracheophytes.