This has puzzled me for a while.

If you're trying to identify a particular plant, often the most 
diagnostic feature is the leaf shape. And that's because plants have a 
dazzling array of leaf shapes. For example,

http://upload.wikimedia.org/wikipedia/commons/e/e8/Leaf_morphology.svg

The fact that humans even needed to invent this many words to describe 
leaf shapes tells you that there's a hell of a lot of leaf shapes out there.

This is puzzling because leaves all do the exact same job. (Well, the 
leaves that haven't been turned into petals or insulation or fly traps 
or some other specialised structure, anyway.) Given that all leaves 
perform the same task, you would expect convergent evolution to result 
in identical leaves. But this hasn't happened. Quite the opposite, in fact.

In particular, the purpose of leaves is to absorb as much sunlight as 
possible. Given that task, you would /expect/ leaves to evolve to be 
huge flat sheets. You would /not/ expect leaves to evolve holes and cuts 
in them. You certainly wouldn't expect to see long, thin, pointy leaves 
which hardly soak up any light at all.

But plants of course are not designed by human thinking. They are 
designed by a far more reliable process: trial and error. The vast 
number of plants with weird shaped leaves demonstrates beyond question 
that this /works/, and that it provides some sort of advantage. But... what?

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