'a versus ('a)

I have seen both 'a and ('a) in type definitions like this:

type 'a tree = Leaf of 'a | Node of 'a tree * 'a tree;;

type ('a) tree = Leaf of 'a | Node of 'a tree * 'a tree;;

It seems they are equivalent. 

Is there a reason why I would write ('a) instead of just 'a? Or is is
just a matter of taste?

-- 
Manfred