... " the term “tree” refers to their lifestyle, not their ancestry. To put it another way, beings that we call trees have evolved several times from different ancestors, whereas beings like ciliates, grasses or (for that matter) primates have evolved only once. Palms evolved into trees independently from species like oaks, for example. Moreover, plant species that exist as herbs or shrubs on a continent often evolve into trees when they find themselves on islands."
http://opinionator.blogs.nytimes.com/2010/04/06/tree-mendous/?ref=science