Bryophytes : 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 cycle is summarized in the fig below.
The haploid generation is called thegametophyte 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 undergoes asexual 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. The figure given above summarises the life cycle of a typical Bryophyte. 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.