Subject: Why are the stems of the leaves of the tree aspen rectangular?

I have noticed that the leaves of the tree aspen always are in 
motion, even in the weakest wind. It is easy to realize that the 
motion is due to that the stem is flat, not round as on "normal" 
trees. It takes more mass, i.e. energy, to build a rectangular 
stem than building a round one, to the same strength. That is 
one disadvantage with squared stem. What is the purpose of the 
squared stem, is it to move the leaves, or what? What is the 
advantage of moving the leaves like this, and having a squared 
stem? What is the root-cause in the environment that have caused 
this evoution? Why haven´t other trees followed, or perhaps they 
have? 

Re: Why are the stems of the leaves of the tree aspen rectangular?