222 LEAVES 



Under these conditions it is not surprising that botanists 

 came to look upon transpiration as a sort of necessary 

 evil in plant life, especially after it was found, as you have 

 noted, that the entrance of solutes into the plant does not 

 necessarily depend upon the entrance of water at the same 

 time. 



b. The advantages of transpiration have been emphasized 

 by certain recent discoveries. It has been found that 

 transpiration, though a menace to the life of the plant in 

 one way, is a protection to its Ufe in another way. Since 

 evaporation is a cooling process, it often prevents leaves 

 from becoming overheated; it may cause their death by 

 drought, but it may also prevent their death by heat. That 

 this death from heat is a real danger is shown by the scald- 

 ing of leaves ; this sometimes occurs when bright sunshine 

 follows quickly after a summer shower which has filled 

 the air with moisture. Under these conditions transpira- 

 tion is checked. The internal temperature of the leaf 

 increases, and, in the absence of transpiration enough to 

 carry off this heat, the leaf is sometimes killed. The heat 

 which may thus kill the leaf is derived from the sunlight. 

 It has been found that only a very small part of the energy 

 which the leaf absorbs from hght is used in photosynthesis;* 

 it has been found that the unused portion of this energy, 

 being transformed into heat, may be very injurious to the 

 leaf if transpiration is checked too much. It is transpira- 

 tion which rids the leaf of this dangerous excess of heat. 

 It has been found in the case of some leaves that the excess 

 of heat, if transpiration be stopped, may raise the internal 

 temperature of the leaf to the death point in even less than 

 ten minutes. 



Succulent plants of the deserts, such as cactuses, have 



