196 



ELEMENTARY STUDIES IN BOTANY 



(transpiration), so that the loss by the leaves represents the 

 largest amount of loss in an ordinary plant. It has been 

 stated that the epidermis with its cuticle checks transpira- 

 tion, but it does not prevent it entirely. The greatest 

 amount of transpiration, however, is by way of the stomata, 



for the water vapor 

 of the internal at- 

 mosphere, obtained 

 from the cells, is 

 diffusing continually 

 into the external 

 atmosphere. If the 

 stomata are closed 

 by the guard-cells 

 and the internal at- 

 mosphere becomes 

 saturated with w r ater 

 vapor, the loss of 

 water from the work- 

 ing cells is very little 

 or none at all. It 

 is evident that the 

 larger the air-spaces 

 in the leaf, that is, 

 the looser the leaf is 

 in texture, the greater 

 is the amount of 

 internal atmosphere, 

 and the more rapid 

 is transpiration. Hence the amount of transpiration from 

 a leaf depends more upon its structure than upon the extent 

 of its exposed surface. 



A simple experiment should be performed to demonstrate 

 the fact of transpiration by placing a glass vessel (bell jar) 

 over a small active plant, care being taken to shut off the 



FIG. 160. Transpiration experiment : a potted ge- 

 ranium sealed with a rubber cloth and covered 

 with a bell jar; the mist and droplets of water on 

 the glass more or less obscure the plant. 



