Stomatal Distribution?

Question

I know there are more stomata on the bottom part of the leaf than the top, and more in the middle than on the side. However, i am not sure why. Could someone explain it to me?

Answer 1

Part of the reason stomata are found on the underside is thought to keep them out of direct light so there is less water loss and slower air movement. However I believe a valid arguement is water repellency. Both dew and rainfall keep a  leaf wet for significant portions of the year in many habitats. Further CO2 diffuses 10^4 times slower across water than across air to the stomata.  If a leaf surface was wet the film of water would esentially block CO2 movement drastically reducing a plants photosynthetic carbon. Causing the water to bead on a leafs surface keeps the pores clear and allows photosynthesis to continue in wet conditions. The most nonwettable leaf surface has the most stomata in 50 of 57 species tested.The most unwettable region is located in the middle where gravity will help pull the water away from the clustered stomata to the leaf edges so it can drip off.

In alpine plants leaves often have furrows. If stomata appear inside the furrow they are raised from the surface but in most cases the stomata are not found within the water channelling furrows. This feature like leaf folding, leaf rolling are adaptations to protect the leaf underside from excess surface wetting. Even leaf hairs can aid in both slowing air movement across a leaf's surface thus slowing water loss and in protecting stomata from wetness in damp or foggy weather. Water films cover the woolly surface while the leaf underneath remains dry.