Evolution of Plants

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.

Lycophytes.

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.

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.

Angiosperms.

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

Bibliography

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.