Bryophytes, or "moss plants" (the phylum includes both mosses and liverworts), are the most primitive of the terrestrial plants and require a moist environment for their existence. They are smaller than tracheophytes and lack the true stems, leaves, and roots that are found in higher plants. Bryophytes are dependent upon the moisture in their surroundings for both reproduction and fluid transport. The flagellated male gametes of bryophytes, like those of their aquatic ancestors, rely on water to help them move toward female gametes. Additionally, because they lack the efficient system of internal fluid transport found in tracheophytes, bryophytes require environmental moisture to ensure that all parts of the plant remain nourished.
Like all plants, the bryophyte life cycle goes through both haploid (gametophyte) and diploid (sporophyte) stages. The haploid gametophyte comprises the main plant (the green moss or liverwort), while the diploid sporophyte is much smaller and is attached to the gametophyte. The haploid stage, in which a multicellular haploid gametophyte develops from a spore and produces haploid gametes, is the dominant stage in the bryophyte life cycle. This stands in direct contrast to the tracheophyte life cycle, in which the diploid stage is dominant. For more information about the life cycles of bryophytes and tracheophytes, see Alternation of Generations.
Together, mosses and liverworts comprise most of the phylum Bryophyta. Each moss sporophyte consists of a foot, stalk, and sporangium-containing capsule that extends outward from the "leafy" gametophyte plant. These sporophytes are somewhat parasitic toward the adult gametophytes to which they are attached, even though these sporophytes are able to produce much of their own food through photosynthesis.
Liverworts gained their name from the liver-like lobes that characterize some types of these plants. Other types are more moss-like, growing on damp rocks or soil, and others are "leafy" and live on moist tree surfaces. Liverworts, which have a life cycle similar to that of mosses (but with simpler sporophytes), can also reproduce asexually through small structures called gemmae that are formed on the surface of the liverwort leaf. After these gemmae break free from the adult plant, they can give rise to new gametophytes on their own.
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