Why do bryophytes and ferns depend on free water to complete their life cycle?

Bryophytes and ferns need water as the medium to complete their life cycle. The male sex cell of both is flagellated and hence motile. These flagellated male cells combine with the female cells to fertilize.

However, for the process of fertilization, the male cell has to swim and reach the female sex organ. Both bryophytes and ferns require a moist surrounding in order to reproduce.

organisms that lack symmetry are called asymmetrical and belong to phylum porifera/sponges.

organisms with specialized stinging cells belong to phylum cnidaria. stinging cells are called cnidoblast cells.

cnidarians show two morphological forms : polyp (sessile) and medusa (mobile)

filter feeding is observed in porifera members.  

alternation of generation is observed in coelenterata or cnidaria. there is alternation of asexual polyp form and sexual medusoid form. it is also referred as metagenesis.