ADVANTAGES OF TRANSPIRATION 263 



water from both trees and Grass. These estimates show the 

 importance of maintaining for plants a suitable supply of moisture 

 in the soil. 



Conditions Affecting Transpiration. — The humidity of the 

 air, temperature, light, and velocity of wind influence trans- 

 piration. 



The Humidity of the air is an important factor in transpira- 

 tion. Other conditions remaining constant, transpiration, in 

 general, increases with the dryness of the air. For this reason 

 hay cures quickly when the atmosphere is dry. It is also during 

 hot days when the air is dry that plants are most likely to wilt. 



Since heat hastens evaporation, transpiration usually rises with 

 the temperature of the surrounding air. Also light, such as the 

 bright sunshine that is common on hot days, is an important 

 factor in raising the temperature of leaves, which thereby have 

 their transpiration increased. In bright sunlight, a large per 

 cent of the light absorbed by leaves is changed to heat, which may 

 raise the temperature of the leaf to 10° or 15° C. higher than the 

 temperature of the surrounding air; and this surplus of heat 

 induces a more rapid vaporization of the water within the leaf. 



The velocity of the wind is an important factor in transpira- 

 tion; for it is well known that the movement of the air has an 

 important effect on the rate of evaporation. Thus wind moving 

 30 miles an hour evaporates water about 6 times as rapidly as 

 calm air. It is for this reason that muddy roads dry more rapidly 

 on windy days. When the air is calm, the air about the plant 

 becomes more nearly saturated and consequently ceases to take 

 water from the plant so rapidly; but when the air is dry and 

 rapidly moving, the plant is constantly enveloped in dry air 

 which permits very little diminution in the rate of transpiration. 

 When winds are both hot and dry, they are very destructive 

 to plants. The dry hot winds of some of the Western states 

 sometimes rob plants of water so rapidly that crops are killed in 

 a few hours. 



Advantages of Transpiration. — Transpiration is an advantage 

 to the plant in two ways. First, it is an important factor in main- 

 taining the flow of water and dissolved substances from the roots 

 to the leaves and other portions of the shoot. Second, by lower- 

 ing the temperature of plants, it often prevents injury from 

 excessive heat. 



