Why do nonvascular plants have larger gametophytes?
There is probably no absolute answer to this – here is the way I think of it. The gametophyte is haploid. Despite the disdavantages from a genetic perspective, there are also advantages – it is less costly in both time and energy to replicate a haploid genome and before the arrival of sexual reproduction in the plant world mitosis served as the means of producing the next generation. Obviously early plants were very successfu,l able to colonise new environments rapidly and probably at high numbers. So that is the basic form of the ancestral type, asexual and haploid. Then add onto that sexual reproduction. Gametophytes continued to use mitosis but were producing gametes rather than whole new gametophytes and this introduces a diploid phase into the life history – the sporophyte which reproduces sexually by means of meiosis. Meiosis is energetically more costly than mitosis as two divisions are required and slower. It would be feasible to suggest that the sporophyte invests its resource
