Ferroseed is based on a bur (http://en.wikipedia.org/wiki/Bur). Ferrothorn is probably a combination of a bur and a stinging nettle (http://en.wikipedia.org/wiki/Stinging_nettle), or any other manner of spiny plants/fruits/seeds.
Oh ok, that is very cool! That is a very interesting plant to base a Pokemon on. I remember getting those stuck on my clothes all the time.