Bulbasaur is a toad-like quadruped that houses a symbiotic plant on its back from birth. Both organisms have evolved to depend totally on the other for survival. The plant, once linked the nervous system of its host, gains mobility and steady access to water and nutrients. The hostis afforded protection by the plant's formidable array of toxic spores and muscular vines.
I Asit grows, the plant on Bulbasaur's back begins to assume control. taking on a greater share of the neural load while the host's brain functions decrease. This effect is compounded in later evolutionary stages, changing the creature's behavior considerably.
Bulbasaur are solitary creatures. They rarely socialize with others of their species, and are territorial in the wild.
There is no account of a Bulbasaur surviving past infancy without its parasite, and subsequently no evidence that the tumor-like bulb can persist without a host.
Pokenatomy by Christopher Stoll