Nitrogen in the atmosphere must be "fixed" before it can be used by plants. atmospheric nitrogen is "fixed" by

nitrogen fixing bacteria

Explanation:

Nitrogen in the forms of nitrates, nitrites and ammonia are essential for the growth of the plants. Nitrogen present in the atmosphere is fixed in the soil by the nitrogen fixing bacteria. As plants cannot absorb nitrogen from the air. The bacteria convert atmospheric nitrogen into inorganic forms such as nitrates, nitrites and ammonia which are directly absorbed by the plants by their roots.

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.

rain is the best answer