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

I asked a lot of people but could not find a reasonable cause for this 
feature. Some plants present these unusual forms but nobody knows why. It 
is just a feature "programmed" in the development of the plant. Usually it 
is a characteristic of the family. This "little stem" is actually the 
petiole, but some plants really have a square stem.
Yes, it creates a tension in the structure of the organ but apparently it 
is not a great problem. It may not be of great ecological advantage, but, 
on the other hand, is not a disadvantage. It just happened to be there. The 
mechanism to create this structure is simple: al the plant has to do is 
grow faster at some parts (the corners) and the non-isodiametric growth 
appears. This structure causes the petiole to present different resistance 
in different directions and it really may explain the movement of the 
leaves.
I am not saying that Darwin was wrong. Evolution does occur and it TENDS to 
create optimised structures. But non-Darwinian evolution also occurs and, 
in these cases, the plant changes with no apparent reason or purpose. It 
just happened. Of course it is not a definitive explanation and a lot of 
people would disagree, but that is another long question...