54 ELEMENTS OF PLANT ANATOMY. 
of turgor it closes. They differ from the other epidermal cells 
not only in shape but also in contents, being richly supplied 
with chlorophyll, which is seldom found in the ordinary epider- 
mal cells. This fact is supposed to have some connection with 
their ability to open and close, or in other words with their 
condition as regards turgor. 
While this is true of stomata in general, there are those 
whose only office is to permit the passage cf water in a liquid 
state. Such are found on leaves of Aroideae and of the genus 
Tropaeoleum, and are called water pores. On leaves of certain 
plants the ordinary stomata are used for this purpose for a time ~ 
before they assume the function of exchange of gases, for 
example, on the leaves of grasses. On the leaves of some 
water plants, partially developed stomata are found which 
appear to be entirely functionless. Certain low plants, the 
thalloid Hepaticae, are furnished with stomata which are very 
peculiar in structure and origin and have no power of motion. 
MA 
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Epithel of petal of Viola tricolor. p papillae with striped cuticle. 300. —(Wiesner.) 
As a general rule, stomata are found in greatest numbers 
on the under surface of leaves; here they vary greatly in 
number, averaging from one to two hundred to the square 
millimeter. Sometimes they reach as high as 700, while on 
