How Substances are Transported and Removed 207 



the relatively dry atmosphere outside. Such is the structural 

 and physical basis of transpiration, and it explains perfectly why 

 heat, which is an evaporation accelerator, and dryness and winds, 

 which are diffusion promoters, increase the process. 



But though such is its basis, transpiration is really not so 

 simple as this, for it is influenced much by another condition, 

 and that is the number and size of the stomata. As to their num- 

 ber, that varies immensely with different kinds of plants, there 

 being none at all on the upper surface of a good many leaves, 

 while on lower surfaces they vary from a few up to near 500 to 

 every square millimeter (one-twenty-fifth of an inch), with a 

 conventional mean at 100; and this equals no less than 100 mil- 

 lions to the square meter (yard), which is another of our con- 

 ventional constants. And it is worth while to add that when all 

 of the stomata are open their widest, about one-hundredth of the 

 whole area of the leaf is exposed. As to the size of the stomata, 

 that not only varies with the kinds, but in each kind is highly 

 variable, since they open and close, from near a circle through a 

 narrowing oval to a slit and perhaps no passage at all, by the 

 movements of two bordering cells called guard cells. These 

 guard cells, as shown by the typical example pictured herewith 

 (figure 71), are of aspect distinctive and unmistakable, with 

 little resemblance to others of the epidermis. They are usually 

 somewhat kidney-shaped, forming together two halves of an 

 elongated oval, and they contain chlorophyll. Their construction 

 is such, as figure 71, lower, illustrates, that the natural spring of 

 their walls tends to bring them together and close up the stomatal 

 slit ; but the development of osmotic turgescence in their cavities 

 rounds them out so that they separate, thus opening the slit. 

 Now this turgescence of the guard cells is influenced much by the 

 quantity of water contained in the leaf, rising and falling there- 

 with, so that when water is plenty the stomata tend to be open, 

 but when it is scarce they tend to be closed. Thus it seems as if 

 the guard cells ought to act adaptively as regulators of transpira- 



