Re: Number of stomata in desert plants vs. in plants in a humid environment

You would expect that stomatal density would be lower in dry environments to 
lower water loss. However, there are several factors that may complicate this 
relationship. One is that stomatal density varies widely from species to 
species. For example, the table in Meyer et al. (1973) gives a low of 1,400 
stomata per square centimeter for wandering jew (Zebrina pendula) to 103,800  
stomata per square centimeter for scarlet oak (Quercus coccinea). 
Unfortunately, none of the plants Meyer et al. (1973) list seem to be typical 
plants of dry environments.

Many plants of dry environments have other mechanisms than low stomatal 
density to reduce water loss such as thick cuticles, photosynthetic stems 
instead of leaves (e.g. cactus), sunken stomata, stomatal hairs, leaves that 
drop during droughts, ephemeral habit and stomata that open only at night, 
i.e. CAM photosynthesis.
 
A typical cactus with spines instead of photosynthetic leaves would almost 
certainly have a much lower total number of stomata than a tree of similar 
fresh weight growing in a moist environment. The cactus also have a much lower 
total surface area.

Plants also can alter their stomatal density based on the carbon dioxide level 
in the atmosphere.

References


Re: What relationship is there between stomatal density and rate of 
transpiration


Some adaptations of xerophytes


Increasing carbon dioxide and its direct effects on ecology


Re: What relationship is there between stomatal density and rate of 
transpiration

Meyer, B.S. et al. 1973. Introduction to Plant Physiology. New York: D. Van 
Nostrand.