Evolution of Plants
Evolution of Plants
Evolution of Plants
Modern classification systems, based largely on molecular evidence, divide living organisms into three domains: Bacteria (also called Eubacteria), Archaea, and Eukarya. Plants are classified as a kingdom (Plantae) within the Eukarya; organisms that possess a nucleus , mitochondria , an internal cytoskeleton , and, in photosynthetic species, chloroplasts. Most scientists recognize three other eukaryotic kingdoms: Protista (most of which are single-celled organisms), Fungi, and Animalia (animals). The fungi, plants, and animals are thought to have evolved from different groups of protists.
Plants are multicellular organisms that have evolved the ability to live on land. The vast majority can carry out photosynthesis, but they are not the only organisms with this ability: many protists can photosynthesize too, as can several important groups of bacteria.
Algae in Plant Evolution
Photosynthetic protists (commonly called algae) are a diverse group of organisms and are divided into several phyla. Many are unicellular, including most euglenoids (phylum Euglenophyta) and dinoflagellates (Dinophyta), and some diatoms (Bacillariophyta) and green algae (Chlorophyta). These, along with the cyanobacteria (often misleadingly called blue-green algae), form the phytoplankton of aquatic ecosystems . Others, including all brown algae (Phaeophyta), most red algae (Rhodophyta), and many green algae are multicellular. The large marine forms of these phyla are usually called seaweeds.
Plants are thought to have evolved from a class of freshwater green algae called the charophytes. Two particular groups of charophyte, the Coleochaetales and the Charales, resemble the earliest land plants (bryophytes) in a variety of ways, including the structure of their chloroplasts and sperm cells, and the way their cells divide during mitosis .
The Importance of Vascular Tissue
Plants are classified into two main groups: the bryophytes (nonvascular plants) and the tracheophytes (vascular plants). Both groups have multicellular embryos, which indicates that they are closely related to each another and distinguishes them from the green algae. Indeed, true plants are often referred to as embryophytes because of this feature. The bryophytes consist of the liverworts, hornworts, and mosses, and as their name implies none of these plants possess vascular tissues.
All other plants, including the ferns, gymnosperms, and angiosperms, are classified as tracheophytes. These possess specialized vascular tissues—phloem and xylem —to transport sugars, water, and minerals throughout their bodies. The oldest known vascular plants appeared in the middle Silurian period (439–409 million years ago); the oldest known bryophytes appeared later, in the Devonian (409–354 million years ago). Despite this, most scientists believe that bryophytes evolved before vascular plants, and that the earliest bryophytes have not been found because they fossilize poorly. This belief is supported by a variety of evidence, including morphological traits, ultrastructural features visible under the electron microscope, and molecular information obtained from gene sequencing.
Since bryophytes are land plants, they need to support themselves in air. However, because they lack lignified vascular tissues, this support must be provided largely by the turgor pressure of their cells. Consequently, they cannot grow to be very tall, and most bryophytes are small and rather inconspicuous. An additional important feature of their lifestyle is their reproductive system. The male gametes , produced by reproductive structures called antheridia, are free-swimming sperm cells that need water to transport them to the female gametes, which are enclosed within structures called archegonia. Because of the need for water, bryophytes are especially common in wet habitats such as bogs, streambanks, and in moist forests. However, they are not restricted to these habitats, and some mosses thrive in deserts, above the treeline, and in the Arctic tundra.
Among the living bryophytes, liverworts are probably most closely related to the earliest land plants, since unlike hornworts, mosses, and all vascular plants they do not possess stomata . Indeed, the fact that stomata first appeared in hornworts and mosses is evidence that vascular plants evolved from one of these two groups. Vascular plants appear to be more closely related to mosses than to hornworts, because some mosses possess food-conducting cells (leptoids) and water-conducting cells (hydroids) that resemble the phloem and xylem of vascular plants.
Early Vascular Plants
The first detailed vascular plant fossils appear in rocks from middle Silurian, about 425 million years ago. The oldest of these, including a plant called Aglaophyton, appear to have possessed conducting cells similar to the hydroids of mosses. These ancient plants, which are sometimes called prototracheophytes, may have been an evolutionary link between the bryophytes and the true tracheophytes. Early vascular plants possessed two features that made them especially well adapted to life on land. First, their vascular tissues transported sugars, nutrients, and water far more efficiently than the conducting cells of mosses. Second, they evolved the ability to synthesize lignin , which made the cell walls of their vascular tissues rigid and supportive. Taken together, these features allowed them to grow much larger than their bryophyte ancestors and considerably reduced their dependence on moist habitats.
There are three major groups of tracheophytes: seedless vascular plants, gymnosperms, and angiosperms. Since the first appearance of tracheophytes in the Silurian, the fossil record shows three major evolutionary transitions, in each of which a group of plants that were predominant before the transition is largely replaced by a different group that becomes predominant afterward. The first such transition occurred in the late Devonian, approximately 375 million years ago. Prior to this time the most common plants were simple, seedless vascular plants in various phyla, several of which are now extinct. However, one phylum from this time, the Psilophyta, still has two living genera, including a greenhouse weed called Psilotum.
From the late Devonian until the end of the Carboniferous period (290 million years ago) larger, more complex seedless plants were predominant. The main phyla were the Lycophyta, the Sphenophyta, and the Pterophyta. All three groups contain living relatives, including club mosses (Lycopodiaceae) in the Lycophyta, Equisetum (the only living genus of sphenophytes), and ferns, which are pterophytes. Only the ferns, which have about 11,000 living species, are common today, but in the Carboniferous these three phyla comprised a large fraction of the vegetation on the planet. Many grew to the size of trees and dominated the tropical and subtropical swamps that covered much of the globe at this time.
The second major transition was the decline of the lycophytes, sphenophytes, and pterophytes at the end of the Carboniferous and their replacement by gymnosperms in the early Permian. Gymnosperms dominated the vegetation of the land for the next 200 million years until they themselves began to decline and were replaced by angiosperms in the middle of the Cretaceous. Although one group of gymnosperms (the conifers) is still abundant, the angiosperms have been the most diverse and widespread group of plants on Earth for the last 100 million years.
The gymnosperms probably evolved from an extinct phylum of seedless vascular plants, the progymnosperms, that appeared about 380 million years ago. The fossils of these plants, some of which were large trees, appear to form a link between the trimerophytes (another extinct phylum of seedless vascular plants) and true gymnosperms. Progymnosperms reproduced by means of spores like the former, but their vascular tissues were very similar to those of living conifers. The oldest true gymnosperms, which produce seeds rather than spores, first appeared about 365 million years ago. The evolution of seeds, with their hard, resilient coats, was almost certainly a key factor in the success of the group. A second factor was the evolution of pollen grains to protect and transport the male gametes. As a consequence of this, gymnosperms, unlike seedless vascular plants, were no longer dependent on water for successful fertilization and could broadcast their male gametes on the wind.
Several early gymnosperm groups are now extinct, but there are four phyla with living representatives: the cycads, the gnetophytes, the conifers, and one phylum (Ginkgophyta) that has only a single living species, the ginkgo tree (Ginkgo biloba ). Of these, the conifers are by far the most abundant and diverse, and many species are of considerable ecological and economic importance. Most conifers are well adapted to dry environments, particularly in their leaf morphology , and some can withstand severe cold. These features may have enabled them to thrive in the Permian, when Earth became much drier and colder than it had been in the Carboniferous.
The angiosperms, or flowering plants, are all members of the phylum Anthophyta. There are at least 250,000 species, making the group easily the most diverse of all plant phyla. They share a number of features that distinguish them from other plant groups. The most obvious of these is the possession of flowers, highly modified shoots that carry the male and female reproductive structures. They also carry out a process called double fertilization, in which two male gametes (sperm nuclei) are released from the pollen tube into the ovule . One of these sperm nuclei fuses with an egg cell in a similar way to gymnosperms. The second nucleus (which degenerates in most gymnosperms) fertilizes other cells in the ovule called polar nuclei. Most commonly, two polar nuclei fuse with the sperm nucleus to form a triploid endosperm nucleus. The tissue that forms from this fusion is called endosperm, which in most angiosperms provides nutrients for the developing embryo.
A third feature that separates angiosperms from gymnosperms is that angiosperm embryos are protected by an ovary wall, which develops into a fruit after fertilization has taken place. In contrast, gymnosperm embryos are held relatively unprotected on the surfaces of ovule-bearing scales in the female cones.
Angiosperms first appear in the fossil record about 130 million years ago, and by 90 million years ago they had become the predominant group of plants on the planet. English naturalist Charles Darwin considered the sudden appearance of angiosperms to be an "abominable mystery," and scientists have debated about the origin of the group for many years. Comparative studies of living species suggest that angiosperms evolved from the gnetophytes, a group of gymnosperms with three living genera of rather strange plants: Ephedra, Gnetum, and Welwitschia. Double fertilization has been shown to occur in both Ephedra and Gnetum, and the reproductive structures (strobili) of all three genera are similar to the flowering stalks of some angiosperms. Some gene sequencing studies also indicate that gnetophytes and angiosperms are closely related to each other and to an extinct group of gymnosperms called the Bennettitales. However, more recent molecular studies suggest that gnetophytes are more closely related to conifers than they are to angiosperms.
In 1998, the discovery of an angiosperm-like fossil called Archaefructus, which apparently existed 145 million years ago, also cast some doubt on the idea that angiosperms descended from gnetophytes or Bennettitales. Although a great deal of information has been obtained since the time of Darwin, the origin of angiosperms is still something of a mystery.
Early Angiosperms, Monocots, and Eudicots
The oldest known angiosperms were a diverse group of plants called magnoliids. Some of these were herbs with simple flowers; others were woody plants with more complex flowers that were very similar to living magnolias. Magnoliids, probably those with small, inconspicuous flowers, gave rise to the two main groups of angiosperms, monocots and eudicots , although a few angiosperm families, including the water lilies, may have evolved earlier.
These plants possessed a number of adaptations that were probably crucial to their eventual success. Their vascular tissues were particularly efficient, their embryos were enclosed in a protective seed coat, their leaves were resistant to desiccation , and they were pollinated by insects, rather than by the wind. This last feature made pollen transfer much more efficient and was almost certainly a key innovation in the diversification of the group, as coevolution of plants and their pollinators, particularly bees, gave rise to increasing specialization of both flowers and insects.
The orchid family contains some of the most specialized insect-pollinated flowers of all and has more species (at least 24,000) than any other plant family. Other groups of angiosperms re-evolved the ability to be pollinated by wind. One of these groups—the grasses—appeared about 50 million years ago, diversified rapidly, and became the dominant plants over many regions of the planet. They still thrive and are crucial to human well-being. Approximately 54 percent of the food eaten by people is provided by grain (seed) from cultivated varieties of just three grasses: rice, wheat, and corn.
see also Algae; Angiosperms; Archaea; Bryophytes; Conifers; Cyanobacteria; Eubacteria; Eudicots; Fruits; Fungi; Gymnosperms; Monocots; Photosynthesis; Plant; Protista; Pteridophytes; Seedless Vascular Plants; Seeds
Simon K. Emms
Friedman, W. E., and S. K. Floyd. "The Origin of Flowering Plants and Their Reproductive Biology." Evolution 55 (2001): 217–231.
Iwatsuki, K., and P. H. Raven, eds. Evolution and Diversification of Land Plants. Berlin, Germany: Springer-Verlag, 1997.
Kenrick. P., and P. R. Crane. "The Origin and Early Evolution of Plants on Land." Nature 389 (1997): 33–39.
Pryer, K. M., et al. "Horsetails and Ferns Are a Monophyletic Group and the Closest Living Relatives to Seed Plants." Nature 409 (2001): 618–622.
Qiu, Y. L., et al. "The Earliest Angiosperms: Evidence from Mitochondrial, Plastid, and Nuclear Genomes." Nature 402 (1999): 404–407.
Raven, Peter H., Ray F. Evert, and Susan E. Eichhorn. Biology of Plants, 6th ed. New York: W. H. Freeman and Company, 1999.
Stewart, W. N., and G. W. Rothwell. Paleobotany and the Evolution of Plants, 2nd ed. Cambridge: Cambridge University Press, 1993.
Sun, G., et al. "In Search of the First Flower: A Jurassic Angiosperm, Archaefructus, from Northeast China." Science 282 (1998): 1692–1695.
Evolution of Plants
Evolution of Plants
Plants, descended from aquatic green algal ancestors, first appeared on land more than 450 million years ago during or prior to the Ordovician period. This event preceded the colonization of land by four-footed animals (tetrapods), which occurred considerably later in the Devonian period (408 to 360 million years ago). Understanding the origin of plants is important because early plants were essential to the development of favorable terrestrial environments and provided a source of food for animals. In addition, the earliest plants were ancestral to all of the food, fiber, and medicinal plants upon which modern humans depend. The hominid lineage , leading to modern humans, is only about 4 million years old; most modern plant community types are considerably older.
Ancient, microscopic fossils and deoxyribonucleic acid (DNA) evidence indicate that the earliest land plants resembled modern bryophytes, the liverworts, hornworts, and mosses. Bryophytes are smaller and simpler than other plants. Other larger and more complete fossils reveal that plants became increased in size, and their structure and reproduction became much more complex during the Silurian and Devonian periods (438 to 360 million years ago). The ancestors of today's vascular and seed plants appeared during this time. During the Carboniferous period (360 to 286 million years ago) the warm, moist climate favored the growth of extensive, lush forests of ferns and other tree-sized vascular plants. These forests had a dramatic effect on Earth's atmospheric chemistry, resulting in a large increase in oxygen and a drastic reduction in carbon dioxide. The consequent reduction in greenhouse warming caused the climate to change to cooler, drier conditions in the Permian (286 to 248 million years ago), and fostered the rise of the seed plants known as gymnosperms . The gymnosperms continued to dominate through the Mesozoic era (248 to 65 million years ago), providing sustenance for giant, herbivorous dinosaurs. Although flowering plants, known as angiosperms, were present by the Cretaceous period (144 to 65 million years ago) and were quite diverse late in this time frame, they shared dominance with gymnosperms until the famously destructive Cretaceous/Tertiary comet or asteroid impact about 65 million years ago. As a result of this event, many previously successful plant groups (as well as dinosaurs and other animals) became extinct. This created new opportunities for flowering plants, mammals, and birds, which consequently became very diverse.
Much of what we know about the origin and evolutionary diversification of plants comes from molecular systematics , the comparative study of DNA extracted from modern plants. This information allows botanists to construct phylogenetic trees, which are branched diagrams from which evolutionary events can be inferred. Phylogenetic trees can also be constructed from structural data, including information from fossils, in order to understand plant evolution. The study of fossils is important because many groups of extinct plants have left few or no close modern relatives from which DNA can be obtained.
The Origin of Land Plants
DNA, structural, and biochemical evidence has conclusively pinpointed a particular group of freshwater green algae known as the charophyceans as the modern organisms that are most closely related to the earliest plants, and have also revealed important steps in plant evolution. Bodies of the most basic charophyceans are either single-celled or form simple groups of cells. Other charophyceans more closely related to plants, according to DNA data, are more complex in their structure and reproduction. These include Coleochaete and Charales, a group that is commonly known as stoneworts. The comparison of simple to more complex charophyceans has revealed the origin of several important plant attributes, including: cellulose cell walls; intercellular connections known as plasmodesmata ; and the phragmoplast , a specialized system of components necessary for plant cell division.
DNA evidence also marks liverworts as the modern land plants that appeared earliest; liverworts have the simplest plant bodies and reproduction of all plant groups. The ancient microfossils thought to represent the remains of the earliest plants are very similar to the components of modern liverworts. However, the order in which various bryophyte groups appeared is somewhat controversial; some experts argue that hornworts may have come first. Nonetheless, most experts are agreed that mosses are the latest-appearing group of bryophytes and that they are most closely related to vascular plants.
The balance of evidence strongly indicates that all of the modern land plants are derived from a single common ancestor (i.e., they are monophyletic ), and that this ancestor resembled modern Coleochaete and Charales. DNA and other evidence do not support earlier ideas that various modern plant groups evolved independently from different charophycean ancestors. Because modern-day charophycean algae occupy primarily fresh waters, the direct ancestors of land plants are thought to have also been fresh water algae; plants did not arise from ocean seaweeds, as was once thought.
The comparison of Coleochaete and Charales to bryophytes, particularly liverworts, has revealed much about the evolutionary origin of plant features that contributed to the ability of the first plants to survive on land. These include reproductive spores that are covered with a resistant material known as sporopollenin, which allows them to be dispersed in the air without dying. An apical (top) region of young, dividing cells (meristem ) that produces a body composed primarily of tissues, reduces the amount of plant surface area exposed to drying. A multicellular sporophyte (spore-producing) body enables plants to reproduce efficiently on land. In plants, sporophytes are always associated with parental gametophytes (the gamete-producing bodies) for at least some time in their early development, which is known as the embryonic stage. This combination of sporophyte and gametophyte in the life cycle is known as alternation of generations. Plant embryos are able to obtain food from the body of their female gametophytes via tissue known as placenta. A placenta is found at the junction of the embryonic sporophyte and gametophyte bodies in all plant groups. The plant placenta is analogous in location, structure, and function to the placenta of mammals. In both mammals and plants, the placenta increases the ability of the parent to produce more young.
Charophycean algae lack sporophytes, tissue-producing meristems, and walled spores. However, they do have precursor features: sporopollenin (though not produced around spores), regions formed of tissues (though these are not extensive and are not produced by an apical meristem), and a placenta (though this supports development of a unicellular zygote rather than a sporophyte). The plant sporophyte body is thought to have originated from the charophycean zygote. Comparison of charophyceans with bryophytes illustrates the evolutionary concept of descent with modification; features inherited by the first land plants from ancestral charophyceans became modified under the influence of terrestrial environments. Comparative studies of modern charophyceans and bryophytes are needed because no fossils are known that illuminate the algae-to-plant transition, which likely occurred in the early Ordovician or the Cambrian (590 to 505 million years ago) periods.
Plants, including bryophytes and vascular plants, are widely known by the term embryophytes because they all have a multicellular, nutritionally dependent embryo (young sporophyte). Synonyms for embryophytes include the term metaphyta, which corresponds to the term metazoa for members of the animal kingdom. The term plant kingdom has been used in a variety of ways by different experts; some restrict this term to embryophytes, some include green algae, and others include brown and red algae as well.
Diversification of Plants
Sometime after the origin of the first plants, bryophytes diversified into the three main modern lineages (liverworts, hornworts, and mosses) and possibly other groups that have since become extinct. Some experts think that bryophytes diversified during the Ordovician period (505 to 438 million years ago). Others are skeptical, because fossils of bryophytes that are sufficiently intact to be sure of their identity are much younger, occurring after the earliest fossils of vascular plants. This is usually explained as the result of the reduced ability of delicate bryophyte bodies to survive damage and decay after death, and the fact that Ordovician deposits are not as well studied as those of later periods. The DNA evidence that bryophytes appeared before vascular plants is very strong. It discounts earlier beliefs, based on the sparse early fossil record, that bryophytes might be descended from vascular plants.
Origin of vascular plants required three important evolutionary advances: (1) sporophytes became able to grow independently of their parents after the embryonic stage; (2) sporophytes were able to branch; and (3) sporophytes acquired lignin-walled vascular tissues. Lignin is a tough, plastic-like material that is deposited in the walls of vascular plant conducting cells, making them stronger and less likely to collapse.
In contrast to vascular plants, bryophyte sporophytes remain dependent on parental gametophytes throughout their lives. Bryophyte sporophytes are unable to branch, so they can produce only one organ that generates spores, the sporangium. Although many bryophytes possess conducting tissues, these lack lignin in their walls. Modern (and fossil) vascular plants, also known as tracheophytes, have branched sporophytes that at maturity are (were) able to grow independently of gametophytes. Independent growth allows tracheophyte sporophytes to live longer than those of bryophytes. Branching vastly increases reproductive potential because many more sporangia and spores can be produced. Lignified vascular tissues provide a more efficient water supply and greater mechanical strength, giving vascular plants the potential to grow much larger than bryophytes. Woody plants contain large amounts of lignified conducting tissues—the strength and durability of wood derives largely from its lignin content.
The fossil record reveals that there were ancient plants that had many of the features of bryophytes, including absence of vascular tissues, but whose sporophytes were branched and capable of living independently at maturity like those of vascular plants. These plants lived in the late Silurian (about 420 million years ago) and into the Devonian period, then became extinct. Known only as fossils, these plants are described as pretracheophyte (meaning "before vascular plants") polysporangiates (meaning "producing many sporangia"). They are represented by fossils such as Horneophyton and are viewed as possible intermediates between bryophytes and vascular plants. They are also interesting because their sporophyte and gametophyte bodies were of similar size and complexity, in contrast to bryophytes (in which gametophytes are usually larger than sporophytes) and vascular plants (whose sporophytes are larger and more complex than gametophytes).
Fossils show that there were early vascular plants that had primitive lignified conducting cells. Later-appearing fossils and modern vascular plants are known as eutracheophytes because they have more complex conducting cells. Modern vascular plants are thought to be derived from a single common ancestor. The comparative study of fossil and modern vascular plants has been valuable in understanding the evolutionary origin of vascular tissues, leaves, and seeds.
Fossil and DNA evidence indicates that the lycopsids were an early group of eutracheophytes; these include modern nonwoody (herbaceous) plants known as lycophytes (Lycopodium, Selaginella, and Isoetes ) and extinct trees that dominated the coal swamps of the Carboniferous period (360 to 286 million years ago), producing extensive coal deposits. Modern and fossil lycopsids have (had) small leaves with just a single, unbranched vein, which are known as microphylls . It is amazing that the Carboniferous lycopsids were able to grow to such prodigious sizes and numbers since they only had tiny leaves with which to harvest sunlight energy. They did not produce seeds.
Ferns and Horsetails.
Later-appearing plants include ferns, the horsetail Equisetum, and seed plants; these plants have leaves with branched veins. Leaves that have veins that branch, and thus are capable of supplying a larger area of photosynthetic cells, can become quite large and are consequently known as megaphylls. Megaphylls are an important adaptation that allow plants to harvest greater amounts of sunlight energy. Ferns, horsetails, and seed plants, as well as some extinct plants known only as fossils, are grouped together to form the euphyllophytes (meaning plants with true or good leaves ). It is thought that megaphylls might have evolved separately in seed plants and ferns from separate ancestors that both had systems of branches called megaphyll precursors. The processes of planation (the compression of a branch system into a single plane) and webbing (the development of green, photo-synthetic leaf tissue around such a branch system) are evolutionary stages in the origin of leaves that may have occurred independently in ferns and seed plants. This is another good example of descent with modification, and it illustrates the fact that similar changes often occur independently in different plant groups because they confer useful properties (convergent evolution). Leaves of one kind or another are thought to have evolved at least six times, in different plant groups.
Gymnosperms arose from a now-extinct group called the progymnosperms. Progymnosperms are represented by fossils such as Archeopteris, a large forest-forming tree that lived from about 370 to 340 million years ago and had megaphylls. Gymnosperms were dominant during the Permian period (286 to 248 million years ago), a time of cool, dry conditions for which gymnosperms were generally better adapted than many ferns and lycophytes. Adaptations that facilitate survival in cool, dry conditions include leaves that have reduced surface area (i.e., are needle or scale-shaped) and seeds. Reduced leaf surface area helps reduce the loss of water by evaporation. Having seeds reduces a plant's dependence on liquid water to accomplish fertilization during sexual reproduction and allows seed dormancy, the ability of the protected embryo to persist until conditions are favorable for germination. Today, gymnosperms are still quite successful in cool and dry environments, such as forests of high latitudes (taiga) and mountains. There were some ancient seed-producing ferns that do not seem to be related to any modern group. These ferns illustrate independent origin of seeds and the value of seeds as an adaptation.
The origin of modern seed plants was accompanied by the first appearance of embryonic roots (radicles). In contrast, nonseed plants lack an embryonic root, rather, roots arise from the adult stem, often from a kind of horizontal stem known as a rhizome.
The origin of the first flowering plants is not well understood, and it is a topic of great interest to botanists. Progymnosperms and the Gnetales, an unusual group of modern gymnosperms, are thought by some experts to be closely related to angiosperms. However, DNA evidence has cast doubt on the connection to Gnetales. DNA evidence also indicates that the most primitive modern flowering plant is Amborella, a native of the Pacific island New Caledonia. Researchers are working to understand the origin of the unique and defining features of flowering plants, including flowers, fruits, and seeds with endosperm . The evolutionary radiation of flowering plants is associated with coevolution—the coordinated evolutionary divergence of many animal groups, including insects, bats, birds, and mammals. Animals depend on these plants as a source of food and play important roles in carrying spores (pollen) between plants and transporting fruits and seeds to new locations. The extinction of any modern flowering plant could thus potentially cause animal extinctions, and vice versa.
There are at least 3,000 living species of charophyceans, primarily desmids living in peat bogs dominated by the moss Sphagnum. Species of living plants are estimated to include 6,000 liverworts, 100 hornworts, 9,500 mosses, 1,000 lycophytes, 11,000 ferns, 760 gymnosperms, and 230,000 angiosperms. New species are continuously being discovered.
see also Algae; Angiosperms; Bryophytes; Evolution of Plants, History of; Gymnosperms; Phylogeny; Seedless Vascular Plants; Systematics, Molecular; Systematics, Plant.
Linda E. Graham
Graham, Linda E. Origin of Land Plants. New York: John Wiley & Sons, Inc., 1993. ———, and Lee W. Wilcox. Algae. Upper Saddle River, NJ: Prentice-Hall, 2000.
Kenrick, Paul, and Peter R. Crane. The Origin and Early Diversification of Land Plants. A Cladistic Study. Washington, DC: Smithsonian Institution Press, 1997.
Raven, Peter R., Ray F. Evert, and Susan E. Eichhorn. Biology of Plants, 6th ed. New York: Worth Publishers, 1999.