TRANSPIRATION 



173 



tact with the spaces are always moist, due to moisture forced out 

 from the cells. The atmosphere has free passage through the 

 stomata and into the intercellular spaces. The air in the inter- 

 cellular spaces is gradually taking up moisture which passes to 

 the outside by diffusion. There is also an exchange of gases 

 which will be considered later (see page 195). Much of the 



Fig. 99. — Potometer used in demonBtrating transpiration. 



transpiration talces place through the stomata, but it may also 

 take place thi'ough the entire surface of very young leaves and 

 to some extent through other parts of the plant. The number 

 of the stomata varies in the different, species of plants, and in 

 plants of the same species when grown under different conditions 

 of soil moisture, air humidity, light, temperature and other 

 factors. They are usually most abundant on the under surface 

 of the leaves, as shown in the following tables : 



