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Bryophytes develop from multicellular embryos in which sterile,
nonreproductive cells surround and protect the zygote. However, they
lack the vascular tissues that allow vascular plants to grow very tall.
All bryophytes have alternation of generations with the gametophyte stage
dominant. The two major types of bryophytes are the liverworts
(with the related hornworts) and the mosses. Embryophyta: Bryophytes and all higher plants are often called Embryophyta because they develop from embryos in which the zygote is surrounded by a protective layer of sterile, nonreproductive cells. Multicellular sex organs are always present. Bryophytes: Embryophyta without vascular tissues (xylem and phloem). Because they lack vascular tissues or true roots, bryophytes cannot be anchored very strongly in the soil or grow very tall. Neither water nor nutrients can be transported from one plant part to another except by diffusion. This restricts bryophytes to small sizes and moist habitats; it also means that all parts of the plant must carry out their own photosynthesis. Bryophytes probably evolved from green algae. All bryophytes have well-marked alternation of generations, with the gametophyte phase dominant. Liverworts and hornworts: Bryophytes whose gametophytes are mostly flat-lying plants with distinct upper and lower surfaces. Single-celled absorptive rhizoids grow from the lower surface. Sporophytes vary but are generally simple. Mosses: Bryophytes whose gametophytes usually have an erect, stem-like portion surrounded with leafy extensions arranged in a circular pattern. Absorptive rhizoids are often multicellular, with cross-walls. Sporophytes are typically more complex than in liverworts, with a spore-bearing capsule supported by a stalk. |
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