Embryophyta is a major grouping of plants, sometimes known as "land plants," that includes both the non-vascular bryophytes (mosses, hornworts, and liverworts) and the vascular land plants, which are those so familiar with their vascular system and true roots, leaves, and stems, such as the ferns, flowering plants, conifers, and ginkgos.
Embryophytes are characterized by an alternation of generations life cycle, apical cell growth, cuticle, antherida (male gametophyte organs), and archegonia (female gametophyte organs (Paleos 2008). Embryophytes are distinguished from the mostly aquatic algae, which do not develop embryos, nor have true roots, stems, or leaves, whereas the embryophytes do form embryos, and have differentiated stems and leaves, and in the case of the vascular plants, true roots.
The origin of embryophytes, as these multicellular plants arose and conquered the land, was a pivotal event in the history of life on earth. Without embryophytes, there would be no animals or humans surviving on the land portion of our planet. The provide food, habitat, energy, oxygen, protection, and numerous vital other functions for the world's creatures. Human beings also benefit from the aesthetic beauty, medicines, and innumerable products derived from these diverse plants.
Embryophytes are the most familiar group of plants. They include trees, flowers, ferns, mosses, and various other green land plants. All are complex multicellular eukaryotes with specialized reproductive organs. With very few exceptions, embryophytes obtain their energy through photosynthesis (that is, by absorbing light); and they synthesize their food from carbon dioxide.
Traditionally, plants were divided into the two groups of embryophytes (Embryobionta), which do develop embryos, and thallophytes (subkingdom Thallobionta), which do not develop embryos and which historically included both algae and fungi (Palaeos 2008). However, fungi are no longer considered plants, and are placed in their own Kingdom. Note that most members of the thallophyte grouping do undergo alternation of generations, with two alternating generations, but all embryophytes undergo such alternation of generations (Palaeos 2008).
Embryophyta may be distinguished from chlorophyll-using multicellular algae by having sterile tissue within the reproductive organs. Furthermore, embryophytes are primarily adapted for life on land, although some are aquatic (which some assume to be secondarily evolved). Accordingly, they are often called land plants or terrestrial plants.
The following are the synapomorphies of the embryophytes: a life cycle with an alternation of generations, apical cell growth (meristem-like growth organization), antheridia, archegonia, and a cuticle (outer covering used to control water loss on land) (Palaeos 2008).
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.
Subgroups: Bryophytes and vascular plants
There are two major groupings within Embryophyta.
Bryophytes, or nonvascular land plants, includes the mosses (division Bryophyta), hornworts (division Anthocerotophyta), and liverworts (division Marchantiophyta). Originally, the three groups were brought together as the three classes or three phyla within the division Bryophyta. However, since the three groups of bryophytes form a paraphyletic group, they now are placed in three separate divisions. They are grouped together as bryophytes because of their similarity as non-vascular land plants. Algae are also non-vascular, but are not "land plants." Note that bryophytes do require water to propagate, and thus live in water or moist habitats.
Like the vascular plants, bryophytes do have differentiated stems, and although these are generally only a few millimeters tall, they do provide mechanical support. They also have leaves, although these typically are one cell thick and lack veins. However, they lack true roots, with their filamentous rhizomes having a primary function of mechanical attachment rather than extracting soil nutrients (Palaeos 2008).
Vascular plants comprise the other group of embryophytes. These are really the true land plants, distinguished by a vascular system, and also characterized by true stems, leaves, and roots. Vascular plants have specialized tissues for conducting water, with water transport occurring either in xylem or phloem. The xylem carries water and inorganic solutes upward toward the leaves from the roots, while phloem carries organic solutes throughout the plant. Vascular plants include the familiar ferns, clubmosses, horsetails, flowering plants (angiosperms), and conifers, and other gymnosperms. Scientific names for this group include Tracheophyta and Tracheobionta, but neither is very widely used.
Evolution and classification
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. Some classifications, indeed, consider the term Embryophyta at the superphylum (superdivision) level, and include land plants and some Charophyceae in a subkingdom named Streptophyta.
Embryophytes are considered to have developed from complex green algae (Chlorophyta) during the Paleozoic era. The Charales, or stoneworts, appear to be the best living illustration of that developmental step. These alga-like plants 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 informally called bryophytes. They include three surviving groups:
All of the above 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 period. During the Devonian period, they diversified and spread to many different land environments, becoming the vascular plants or tracheophytes.
Tracheophyta have vascular tissues or tracheids, which transport water throughout the body, and an outer layer or cuticle that resists drying out. In most vascular plants, the sporophyte is the dominant individual, and develops true leaves, stems, and roots, while the gametophyte remains very small.
Many vascular plants, however, still disperse using spores. They include two extant groups:
- Lycopodiophyta (clubmosses)
- Pteridophyta (ferns, whisk ferns, and horsetails)
Other groups, which first appeared towards the end of the Paleozoic era, reproduce using desiccation-resistant capsules called seeds. These groups are accordingly called spermatophytes or seed plants. In these forms, the gametophyte is completely reduced, taking the form of single-celled pollen and ova, while the sporophyte begins its life enclosed within the seed. Some seed plants may even survive in extremely arid conditions, unlike their more water-bound precursors. The seed plants 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, among the flowering plants or angiosperms, the pollen has to grow a tube to penetrate the seed coat. Angiosperms were the last major group of plants to appear, developing from gymnosperms during the Jurassic period, and then spreading rapidly during the Cretaceous. They are the predominant group of plants in most terrestrial biomes today.
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