Illustrating Leaf Structure
The leaf at the left presents the upper surface, and that at the right the lower surface. It will be seen that the upper surface is more smooth and glossy, in contrast with the more clearly ribbed and reticulated lower surface. The all-important stomata, or breathing pores, are largely located on the lower side of the leaf.
This image is from: Luther Burbank: his methods and discoveries and their practical application. Volume 7 Chapter 1