Here are two different aspen leaves I collected over the summer. One is a bit smaller (they are at the same scale) but how similar are they otherwise?
What do they look like on the inside? Here are the vein networks for each, shown at the same scale:
The smaller leaf seems to have a higher density of veins than the larger leaf. Why? There are a few possibilities. One is very simple: because the second leaf is smaller. In leaf development, the venation network is laid out early on and fixed for the lifetime of the leaf, whereas leaf size is determined much later. It may be that my second leaf has just not finished expanding, and it has the same fundamental network as the first leaf.
Another explanation is that both leaves are adapted to their respective environments. What I didn’t tell you is that these leaves come from different trees, different clones, different locations – an elevation difference of a few hundred feet and a change from wet forest to rocky talus. Maybe these different leaves function differently – and each, better – in the place they live. In fact, the first leaf has a 15% higher photosynthesis (carbon uptake) rate than the second leaf, but also a 15% lower carbon construction cost. What kind of economic tradeoff is this? Or is this a fluke?
I don’t know which hypothesis (if either) is correct yet. I have another 256 leaves from other aspen trees to also analyze. With the right statistical techniques I should have a confident answer in a few weeks!