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. 
