The Orchard Plant 63 



used in building tissue. This excess must pass off through 

 the stomata. It has been estimated that from 15 to 25 

 pounds >f water must pass through a plant in order to 

 produce one ounce of dry material; or to make it more 

 expressive, in the formation of 1000 pounds of dry matter, 

 240,000 to 400,000 pounds of water are used. 



A German investigator calculated the amount of water 

 evaporated by an oak tree which was about 20 feet high 

 and which grew in an isolated situation. His experiments, 

 the results of which are given in the following table, ex- 

 tended through the growing season : 



TABLE I. AMOUNT OF WATER EVAPORATED FROM AN OAK TREE 



DURING THE GROWING SEASON 



May (14 days) 1,944 pounds. 



June 57,250 pounds. 



July 63,265 pounds. 



August 47,839 pounds. 



September 38,882 pounds. 



October 37,450 pounds. 



Total 236,630 pounds. 



This is equivalent approximately to 29,578 gallons. 

 This investigator also "found that water evaporated 

 during the season, when considered with reference to the 

 area of ground covered by the tree top, was equal to a 

 layer 212 inches high; observation had shown the annual 

 rainfall to be 25.6 inches; so that the water evaporated 

 from the tree was eight times the amount which fell upon 

 the earth under it." * 



The passage of air and gases in and out of leaves is an 

 equally important function of stomata. About one-half 

 of the dry weight of a tree is composed of carbon, or the 

 1 Adapted from Bessey, "Botany," p. 172. 



