Crystal: During the day, it stays in the cold underground to avoid the sun. It grows by bath ing in moonlight.Red: During the day, it keeps its face buried in the ground. At night, it wanders around sowing its seeds.Yellow: It may be mistaken for a clump of weeds. If you try to yank it out of the ground, it shrieks horribly.
Prof. Lotus' Notes: Oddish has a fascinating branching evolution line, and my hunch is that is based on whether it fights off or doesn't fight off a parasitic infection that this Pokémon is susceptible to.