Veins tell you a lot about a plant.
For example, if a plant opens its leaf pores, called stomata, to absorb more carbon dioxide for photosynthesis, the leaf also loses a lot of water to evaporation. That requires lots of plumbing in the leaves to pipe in the water. That, in turn, means lots of big veins.
If a plant requires lots of water all the time, it could favor certain geometrical arrangements of veins, which starts to suggest overall leaf shapes.
So it's the veins -- the skeleton of the leaf -- which determine whether you will have a classic maple shape or a blade-like willow.