Re: Why do leaves have fewer stomates on the top than on the bottom?

Actually, saying that leaves have fewer stomates on the top than on the bottom is only part of the story. True, in many species there are more stomates on the lower surface and in some extreme cases there are no stomates on the upper surface. However, this is not always the case. For example, according to Salisbury and Ross (1992) water lily leaves have lots of stomates on the top surface but very few if any stomates on the lower surface. Some plants, such as grasses, have an equal number of stomates on the upper and lower surfaces of their leaves. Aquatic plants often have no stomata.

There seems to be genetic control of where stomates develop. For various reasons, stomata do not typically develop over veins nor do they seem to develop over certain cell types (Larkin, Marks, Nadeau, and Sack, 1997).

Why would stomata develop where they do? Aside from genetic control, keep in mind that stomates are important for gas exchange but in doing gas exchange the plant also loses water through the stomata. Clearly in the water lily, stomates submerged in the water would be useless. The same holds true for aquatic plants. There is no need for stomates since gas exchange can occur readily across the interface between the plant and the water. Though I haven't found anything supporting this, it makes sense that in many species the number of stomates is greater on the lower surface of the leaf because the amount of water loss from the lower surface will be less. So, presumably, the stomates can stay open for longer in order to allow for gas exchange and the plant will not lose as much water.

Interestingly, the density of stomates which develop in leaves may be tied to the amount of carbon dioxide in the atmosphere. As the amount of carbon dioxide increases there are fewer stomates per unit area of the leaf (Woodward,1987).

I hope this information helps out!

Literature Cited:
Larkin, JC., Marks, MD., Nadeau, J., and Sack, F. 1997. Epidermal Cell Fate and Patterning in Leaves. The Plant Cell 9:1109-1120.
Salisbury, FB and Ross, CW. 1992. Plant Physiology, 4th ed. Wadsworth Publishing Company. p.72.
Woodward, FI. 1987. Stomatal densities are sensitive to increases in CO2 from pre-industrial levels. Nature 327:617-618