A tree is something resembling a tree

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Unlikely as it sounds, I recently got involved in a drunken discussion on the difference between a tree, a bush and a plant.

I Googled 'tree' on my smartphone and one of the first definitions I came across was:

Something constructed in the form of, or considered as resembling, a tree, consisting of a stem, or stock, and branches; as, a genealogical tree.

That's from the 1913 edition of Webster's.

Now I'm sober, I understand what the definition is driving at - that 'tree' is sometimes used metaphorically (or perhaps I mean analogously?).

But I have to say that defining a tree as something "in the form of... a tree" does not help resolve drunken arguments. And really, what else are dictionary definitions for?