- 1) Classification of Division Bryophyta
- 2) Hepaticopsida (Liverworts)
- 3) Bryopsida
- 4) Anthoceropsida (Horn Worts)
- 5) Alternation of generations
- 6) The significance of alternation of generations
Classification of Division Bryophyta
Bryophytes are divided into three subdivisions:
Bryophytes belonging to this subdivision are called liverworts. It includes about 900 species. Liverworts are the simplest of all bryophytes. They are normally found on moist rocks and on wet soil. Since they live near water therefore possibilities of drying are significantly reduced.
Characteristics of Hepaticopsida
- The plant body is a gametophyte. It may be thalloid i.e., flat, or ribbon-like, normally dichotomously branched. It is attached to the 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. The sporophyte depends on the gametophyte for nutrition and protection.
- The sex organs establish on the upper surface of the thallus near the tips of the branches. Sometimes they establish unique branches on gametophyte called the antheridiophores and the archegoniophore as in Marchantia.
Like liverworts, most mosses inhabit damp places. In contrast to other bryophytes, they grow equally well in relatively dry places. However, water is essential in the reproduction of mosses, hence they normally grow to form cushions or mats.
Characteristics of Bryopsida
- Each adult moss plant, a gametophyte, is always differentiated into structures that look like stems and leaves. Multicellular rhizoids are likewise present. Examples of mosses are Funaria and Polytrichum. Archegonia and antheridia, establish 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 sterilized hairs, the paraphyses.
- Formation of diploid sporophyte and haploid spores follow the very same series of occasions of alternation of generations as in liverworts. 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.
Anthoceropsida (Horn Worts)
This group of bryophytes differs in lots of respects and is slightly advanced than Bryopsida and Hepaticopsida. The gametophyte is extremely lobed and irregular in an overview. Except for a little early stage of development, the sporophyte is not dependent upon gametophyte for nutrition and protection. Antheridia and archegonia are partly sunken in the gametophytic tissue.
Characteristics of Anthoceropsida
The sporophyte exhibit lots of advanced characters due to which it can flourish better on land 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 the gametophyte. It likewise has a waxy cuticle to prevent excessive loss of water (desiccation). Furthermore, at the junction of the foot and spore-producing region, there is a band of meristematic tissue.
This tissue keeps including cells towards the spore-producing region throughout the formation, maturation, and dispersal of spores from the opposite end. Due to the fast growth rate of this meristematic tissue the sporophyte keeps increasing in length for an indefinite amount of time. Due to these characters, the sporophyte continues to make it through as such even after the death and decay of the gametophyte.
One fine example of Anthoceropsida is Anthoceros which is also found in the hilly areas of Pakistan.
Alternation of generations
In the life history of liverworts, mosses, and hornworts there are two unique multicellular phases or generations. These generations are haploid gametophyte and diploid sporophyte, which regularly alternate with each other. The gametophyte is the dominant generation due to the fact that it is more conspicuous. It produces gametes called spermatozoids or antherozoids and eggs, therefore called gamete-producing generation. A haploid spermatozoid fuses with a haploid egg to produce a diploid oospore.
The oospore does not produce the gametophyte directly but produces an absolutely different plant called the sporophyte. The sporophyte in bryophytes is a less conspicuous generation, which is normally differentiated into foot, seta, and capsule (likewise 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 however generates 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 biography of a plant is called alternation of generations. It needs to be kept in mind that the gametophyte or haploid stage begins with spores and ends at gametes, whereas the sporophyte begins with oospore and ends at the spore mother cell.
The significance of alternation of generations
Throughout the formation of spores from spore mother cells by meiotic division reshuffling of genes takes place. As a consequence, a great range of spores with different genes are produced. These spores in turn produce gametophytes with different hereditary combinations.
The gametophytes with much better heredity will have a better opportunity for survival in the environment where they take place. On the other hand, the gametophytes with less beneficial attributes 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 new genetics as compared to the parent.
This hereditary variation passes to the new sporophyte which on maturity once again produces additional genetic recombination which is moved to the gametophyte. In this natural process, the sporophyte, therefore, offers a big amount of genetic variability and nature selects the very best genetic combinations. In the long run, this will enable the populations to end up being significantly much better adapted to their environment.