Alternation of Generations

Alternation of Generations:

In general with all land plants and certain advanced algae like Laminaria, bryophytes show alternation of generations. Two kinds of organism that is, a haploid gametophyte generation and a diploid sporophyte generation, alternate in the life cycle. The cycle is summed up in the figure shown below:

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 Figure: Generalized life cycle of a Bryophyte plant exhibiting alteration of generation

The haploid generation is termed as the gametophyte since it undergoes sexual reproduction to generate gametes. The production of gametes includes mitosis; therefore the gametes are too haploid. The gametes fuse to make a diploid zygote that grows into the later generation that is the diploid sporophyte generation. It is termed as sporophyte since it undergoes asexual reproduction to generate spores. The production of spores includes meiosis, and hence there is a return to the haploid situation. The haploid spores give mount to the gametophyte generation.

One of the two generations is always much conspicuous and engages a bigger proportion of the life cycle. This generation is termed as the dominant generation. In each Bryophyte the gametophyte generation is dominant. And in all other land plants the sporophyte generation is dominant. It is habitual to place the dominant generation in the top half of the life cycle figure. The figure given above summed up the life cycle of a usual Bryophyte. The one point that should be remembered here is that gamete production includes mitosis and not meiosis as in animals. Meiosis takes place before the production of spores.

 

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