When olfactory neural cells get to a critical stage or are damaged by nerve injury or are exposed to environmental agents that enter the nasal cavity, they tend to retain their stem cells,they degenerate and are replaced by newly generated receptor cells.
Nitrogen fixation, N2 --> NH3+, is not something that plants to by themselves but some plants have bacteria that does this. I believe that pea plants have these, but I'm not 100% sure.
