In a leaf, are there more stoma at the bottom or at the top of the leaf? Why at wherever they are?

Question

In a leaf, are there more stoma at the bottom or at the top of the leaf? Why at wherever they are?

Answer 1

It is interesting that frequently (in broadleaved plants like tobacco) that the stomatal density is higher on the bottom (shaded) side of the leaf.  It seems possible that this is associated with temperture gradients that could occur when the leaf is in direct sunlight.  Of course, the top of the leaf would tend to be warmer in direct sunlight.  This would tend to cause faster water loss by transpiration.  A lower stomatal density could mitigate this loss.  In spite of the differing stomatal  densities (top and bottom) this does not present a big problem for photosynthesis when the stomata are open at least.  CO2 entering from the underside is able to diffuse throughout the entire thickness of the leaf since there are a lot of air spaces around the cells.