176 COLLEGE BOTANY 



whick gives the leaf a temperature that may occasionally be as 

 much as 10 or 15 degrees C. higher than the surrounding air. 



(J) Transpiration is increased by the movement of the 

 wind. It is estimated that wind moving at a velocity of 30 miles 

 per hour will evaporate water about six times faster than 

 calm air. 



(e) Transpiration is influenced by various leaf modifica- 

 tions. These modifications vary with the factors which we have 

 discussed above. An examination of plants growing under the 

 various conditions presents some very interesting facts, some 

 of which will be given very briefly : 



(1) The corn leaves roll during a drouth. 



(2) The cactus, aloes and many other plants growing under 

 dry conditions have reduced surfaces. 



(3) The yuccas and carnations have the stomata at the 

 lx>ttom of the epidermal pits. 



(4) The pines and many desert plants have very thick 

 cuticles. 



(5) The cabbage leaves have a waxy covering. 



( 6 ) The mullein and many other plants have numerous hairs 

 which check transpiration. 



(7) Many plants, especially those growing in arid regions, 

 have special devices for the storage of water. 



Advantages of Transpiration. (1) Transpiration is one of 

 the factors influencing the movement of water through the plant. 

 The more rapid the transpiration, the greater the movements of 

 water. (2) Transpiration decreases with the lowering of the 

 atmospheric temperature in the same manner as ordinary evap- 

 oration from an exposed surface of water. Under normal con- 

 ditions there is a movement of water from the organs of absorp- 



