There are fossil records of blue green algae (Cyanobacteria) living 3000 million years ago and many eukaryotic organisms have existed for more than 1000 million years. However the first organisms to colonize the land, primitive plants did so only 420 millions years ago. The greatest simple problem to overcome in making the transition from water to land is that of desiccation. Any plant not protected in some way, for example, by a waxy cuticle, will tend to dry out and die very soon.
Salient features of Bryophyta
Bryophyta are the simplest group of land plants. They are relatively poorly adapted to life on land, so they are mainly confined to damp,shady places. These are terrestrial non-vascular plants(no vascular tissue namely xylem and phloem) which still require moist environment to complete their life-cycle. Hence these are called amphibians of plant kingdom. They are more advanced than algae in that they develop special organs. The male sex organ is called antheridium and the female sex organ is called archegonium. Bryophytes show distinct alternation of generation in their life cycles. Bryophytes include mosses, liverworts and hornworts.
Distinguishing features of Bryophytes
1. They are small terrestrial plants.
2. They are without a distinct root system but are attached to the substratum by means of thin, filamentous outgrowth of the thallus called rhizoids.
3. Water and mineral salts can be absorbed by the whole surface of the plant body, including the rhizoids. So the main function of rhizoids is anchorage, unlike true roots (true roots also possess vascular tissues, as do true stems and leaves). Thus the “stems” and “leaves” found in some Bryophytes are not homologous with stems and leaves of vascular plants. The plant body is called thallus.
4. They do not possess true vascular tissues.
5. Male sex organ is called antheridium and female sex organ is called archegonium.
6. Sex organs are multi-cellular and they have a protective jacket layer of sterile cells.
7. Sexual reproduction is of oogamous type.
8. Bryophytes show distinct alternation of gametophytic generation with sporophytic generation.
9. Gametophyte generation is dominant and independent.
10. Sporophyte generation is very small, microscopic and dependent on the gametophyte phase.
Alternation of Generations
In common with all land plants and some advanced algae such as Laminaria, bryophytes exhibit alternation of generations. Two types of organism, a haploid gametophyte generation and a diploid sporophyte generation, alternate in the life cycle.
The haploid generation is called the gametophyte because it undergoes sexual reproduction to produce gametes. Production of gametes involves mitosis, so the gametes are also haploid. The gametes fuse to form a diploid zygote which grows into the next generation, the diploid sporophyte generation. It is called sporophyte because it undergoesasexual reproduction to produce spores.
Production of spores involves meiosis, so that there is a return to the haploid condition. The haploid spores give rise to the gametophyte generation. One of the two generations is always more conspicuous and occupies a greater proportion of the life cycle. This generation is called as the dominant generation. In all Bryophytes the gametophyte generation is dominant. In all other land plants the sporophyte generation is dominant. It is customary to place the dominant generation in the top half of the life cycle diagram. One point that must be remembered here is that gamete production involves mitosis and not meiosis as in animals. Meiosis occurs before the production of spores.