220 



LEAVES 



walls are permeable and which are in contact with air. 

 This air, having entered the stomates, diffuses freely among 

 the intercellular spaces. Under such conditions, unless 

 the air already contains all the moisture it can hold, evapora- 

 tion is sure to occur. The area of the moist cell walls 

 which are exposed to the air in the spongy parenchyma is 



many times as great as 

 the total leaf surface. 

 (See Figure 7p.) With 

 these facts in mind, it is 

 not difi&cult to under- 

 stand the source of the 

 drops of water which 

 collect on the inside of a 

 glass bell jar which is 

 placed over a healthy 

 potted plant. 



Water has more influ- 

 ence in determining the 

 way plants grow and 

 where they grow than 

 any other single factor ; it is usually water which makes 

 all the difference between fertile land and desert land ; it* 

 is the supply of water which, more than anything else, 

 determines the size of the crops of the world. Thus, for 

 example, if varieties of wheat or corn could be made to 

 grow with less water than is usually required for them, 

 millions of acres which are now not used at all could then 

 be used for growing these important crops. Varieties of 

 wheat and corn hardy enough to grow under such con- 

 ditions have been discovered, and experimenters are 

 now at work trying to improve these varieties as to 



Fig. 79. — Cross section of a leaf of lily show- 

 ing the internal structure. 



