Bryophytes are divided into three subdivisions : hepaticopsida, bryopsida and anthoceropsida.
Hepaticopsida (Liverworts)
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.
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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.
Fig. 9.6A generalized life cycle of a bryophyte showing alternation of generation.
Bryopsida
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
Classification Anthoceropsida (Horn Worts)
This group of bryophytes differs in many respects and is slightly advanced than bryopsida and Hepaticopsida. The gametophyte is highly lobed and irregular in outline.except for a little early stage of development, the sporophyte is not dependent upon gametophyte for nourishment and protection. Antheridia and archegonia are partially sunken in the gametophytic tissue. The sporophyte exhibit many advanced characters due to which it can thrive better on alnd as compared to other groups.
The sporophyte has stomata and chloroplasts in the epidermis and can thus photosynthesize its own food rather than obtaining it from ghametophyte. It also has a waxy cuticle to check excessive loss of water (desiccation). Furthermore. It also has a waxy cuticle to check excessive loss of water (desiccation). Furthermore, at the junction of foot and spore producing region there is a band of meristematic tissue.
This tissue keeps on adding cells towards the spore-producing region during the formation, maturation and dispersal of spores from the opposite end. Due to the fast growth rate of this meristematic tissue the sporophyte keeps on increasing in length for an indefinite period of time. Due to these chareacters the sporophyte continues to survive as such even after the death and decay of the gametophyte. One good example of anthoceropsida is anthoceros which is also found in the hilly areas of Pakistan (Fig. 9.9).
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Fig. 9.9Anthoceros, a hornwort (a) Gametophyte with attached horn-shaped sporophyte (b) V.S. of sporophyte.
In the life history of liverworts, mosses and hornworts there are two distinct multicellular phases or generations. These generations are haploid gametophyte and diploid sporophyte, which regularly alternate with each other. The gametophyte is the dominant generation because it is more conspicuous. It produces gametes called spermatozoids or antherozoids and eggs, therefore called gamete-producign generation. A haploid spermatozoid fuses with a haploid egg to produce diploid oospore.
The oospore does not produce the gametophyte directly but produces a totally different plant called sporophyte. The sporophyte in bryophytes is a less conspicuous generation, which is usually differentiated into foot, seta and capsule (also called sporogonium). Spores develop within the capsule by reduction division (meiosis) from spore mother cells.
The sporophyte produces spores and is, therefore, called spore producing generation. The spore on germination does not develop into a sporophyte but gives rise to the gametophyte. Thus in the life-history of a bryophytic plant, the two generations, the gametophyte and the sporophyte, regularly alternate with each other. The phenomenon of alternation of gametophyte and sporophyte in the life history of a plant is called alternation of generations (Fig. 9.10).
Fig. 9.10graphic representation of the alternation of gametophytic and sporophytic generation
It should be noted that the gametophyte or haploid stage begins with spores and ends at gameters, whereas the sporophyte begins with oospore and ends at spore mother cell.
The significance of alternation of generations
During the formation of spores from spore mother cells by meiotic division reshuffling of genes occurs. As a consequence, a great variety of spores with different genetic make-up are produced. These spores in turn produce gametophytes with different genetic combinations. The gametophytes with better genetic make-up will have a better chance for survival in the environment where they occur. On the other hand, the gametophytes with less advantageous characteristics will be eliminated. There is no reshuffling of genes during gametogenesis in the gametophyte as gametes are produced after mitosis.
The oospore developing after fertilization now has a new genetic make-up as compared to the parent. This genetic variation passes to the new sporophyte which on maturity once again produces further genetic recombination which are transferred to the gametophyte. In this natural process the sporophyte thus rovide a large amount of genetic variability and nature selects the best genetic combinations. In the long run, this will allow the populations to become increasingly better adapted to their environment.