Bryophytes are divided into three subdivisions : hepaticopsida, bryopsida and anthoceropsida.
Tryophytes belonging to this subdivision are called liverworts. It includes about 900 species. Liverworts are the siplest of all bryophytes (Fig.9.4).
They are usually found on moist rocks and on wet soil, since they live near water therefore chances of drying out are greatly reduced.
Fig.9.4 (a) Marchantia, a typical liverwort, the gemma cups function in asexual reproduction (b) Porella, a leafy liverwort showing lateral antheridia bearing branch.
The plant body is a gametophyte. It may be thalloid i.e. flat, or ribbon-like, usually dichotomously branched. It is attached to soil by means of rhizoids e.g. marchantia. Other species tend to grow upright and are falsely leafy i.e., differentiated into a false stem, and leaves e.g., porella (fig. 9.4b). the saprophyte is dependent upon gametophyte for nourishment and protection.
The sex organs develp on the upper surface of the thallus near the tips of the branches. Scmetimes they develop on special branches on gametophyte called the antherdiophores and the archegoniophores as in marhcantia (fig. 9.5).
Fig. 9.5A Liverwort, Marchontia bearing sex organs, antheridia and archegonia, on special branches called antheridiophores and archegoniaphores.
Like liverworts most mosses inhabit damp places. In contrast to other bryophytes they grow equally well in fairly dry places. However, water is essential in the reproduction of mosses, thus they usually grow to form cushions or mats.
Each adult moss plant, a gametophyte, is always differentiated into structures which resemble stem and leaves. Multicellular rhizoids are also present. Examples of mosses are funaria and polytrichum (Fig. 9.7). Archegonia and antheridia, develop on the tips of different branches on the same plant e.g. , Funaria, or on different plants as in polytrichum. The archegonia and antheridia form clusters and are mixed with sterile hairs, the paraphyses.
Fig 9.7 polytrichume, a hair cup moss plant.
Formation of diploid sporophyte and haploid spores follow the same sequence of events of alternation of generations as in liverworts (Fig. 9.6). however, the spore of a moss, unlike that of liverworts, develops into an alga like structure, the protonema. Haploid moss plants (gametophyte) develop from buds on the protonema and the life cycle is completed (Fig. 9.8)
Fig. 9.8Moss life cycle
Go To Next Part 2