Originally Posted by Urpriest
Not familiar with PF, but in 3.5 Wild Shape is based on Alternate Form, not Polymorph. Given how PF polymorph is broken into a bunch of different spells, it would be extremely bizarre for the concept of polymorph immunity to continue to exist anyway. I'd chalk it up to a copy-paste error.
Wild Shape in PF is based on the Beast Shape
line of spells, which is a Transmutation (Polymorph) spell. Plants are indeed immune to polymorph in PF.
Edit: Just to explain why this might be useful, spells like Baleful Polymorph
are Transmutation (Polymorph) affects.