In addition to acting through a different mechanism, it also has a slightly different function from mammalian immune memory. Systemic Acquired Resistance is first and foremost a method of raising the immune response throughout the plant in response to a local infection. This is predicated on the fact that plants grow in a modular, only semi-deterministic manner with different organs living relatively independently of each other (at least compared to how most animals' bodies work) and the fact that they don't have a circulatory system that rapidly transports substances between organs.

Such a system wouldn't be useful in mammals, because a pathogen in the bloodstream is going to travel just as fast as any endogenous signal, and (since mammals are motile) an infection at one location in the body is unlikely to be followed by a subsequent infection at another location. In contrast, sessile organisms beset by e.g. a fungal pathogen can expect to be infected at multiple locations over a fairly short period of time.