Embryophyte

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The embryophytes are the most familiar group of plants, including trees, flowers, ferns, mosses, and various others. All are complex multicellular organisms with specialized reproductive organs and, with very few exceptions, they obtain their energy through photosynthesis, i.e. by absorbing light, and synthesize food from carbon dioxide. They may be distinguished from multicellular algae by having sterile tissue within the reproductive organs. Further, embryophytes are primarily adapted for life on land, although some are secondarily aquatic. Accordingly they are often called land plants.

Embryophytes developed from complex green algae during the Palaeozoic era. Their closest living relatives are the Charales or stoneworts. These algae undergo an alternation between haploid and diploid generations, respectively called gametophytes and sporophytes. In the first embryophytes, however, the sporophytes became very different in structure and function, remaining small and dependent on the parent for their entire brief life. Such plants are called bryophytes; they include three surviving groups:

Bryophyta (mosses)
Anthocerotophyta (hornworts)
Marchantiophyta (liverworts)

All bryophytes are relatively small and are usually confined to moist environments, relying on water to disperse their spores. Other plants better adapted to terrestrial conditions appeared during the Silurian, and during the Devonian they diversified and spread to many different land environments. These are called vascular plants or tracheophytes. They have vascular tissues or tracheids, which transport water throughout the body, and an outer layer or cuticle that resists desiccation. In most the sporophyte is the dominant individual, and develops true leaves, stems, and roots, while the gametophyte remains very small.

Many vascular plants still reproduce using spores, including the following extant groups:

Lycopodiophyta (clubmosses)
Equisetophyta (horsetails)
Psilotophyta (whisk ferns)
Ophioglossophyta (adders'-tongues and grape-ferns)
Pteridophyta (ferns)

Other groups, which first appeared towards the end of the Palaeozoic, reproduce using desiccation-resistant capsules called seeds. They are accordingly are called spermatophytes or seed plants. In these forms the gametophyte is completely reduced, taking the form of single-celled pollen and ova, and the sporophyte begins its life enclosed within the seed. Some seed plants may survive in extremely arid conditions. They include the following extant groups:

Cycadophyta (Cycads)
Ginkgophyta (Ginkgo)
Pinophyta (Conifers)
Gnetophyta (Gnetae)
Magnoliophyta (Flowering plants)

The first four groups are referred to as gymnosperms, since the embryonic sporophyte is not enclosed until after pollination. In contrast, the flowering plants or angiosperms the pollen has to grow a tube to penetrate the seed coat. They were the last major group of plants to appear, developing from gymnosperms during the Jurassic and spreading rapidly during the Cretaceous. They are the predominant group of plants in most terrestrial biomes today.

Note the higher-level classification of plants varies considerably. Some authors have restricted the kingdom Plantae to include only embryophytes, others have given them various names and ranks. The groups listed here are often considered divisions or phyla, but have also been treated as classes, and they are occasionally compressed into as few as two divisions.

On a microscopic level, embryophyte cells remain very similar to those of green algae. They are eukaryotic, with a cell wall composed of cellulose and plastids surrounded by two membranes. These usually take the form of chloroplasts, which conduct photosynthesis and store food in the form of starch, and characteristically are pigmented with chlorophylls a and b, generally giving them a bright green color. Embryophytes also generally have an enlarged central vacuole or tonoplast, which maintains cell turgor and keeps the plant rigid. They lack flagella and centrioles except in certain gametes.

References

Kenrick, Paul & Crane, Peter R. (1997). The Origin and Early Diversification of Land Plants: A Cladistic Study. Washington, D. C.: Smithsonian Institution Press. ISBN 1-56098-730-8.
Raven, Peter H., Evert, Ray F., & Eichhorn, Susan E. (2005). Biology of Plants (7th ed.). New York: W. H. Freeman and Company. ISBN 0-7167-1007-2.
Stewart, Wilson N. & Rothwell, Gar W. (1993). Paleobotany and the Evolution of Plants (2nd ed.). Cambridge: Cambridge University Press. ISBN 0-521-38294-7.
Taylor, Thomas N. & Taylor, Edith L. (1993). The Biology and Evolution of Fossil Plants. Englewood Cliffs, NJ: Prentice Hall. ISBN 0-13-651589-4.

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