72 ELEMENTARY BIOLOGY 



cause. And yet transpiration, as this evaporation from the 

 leaves is called, may be of use to the plants indirectly. 



This rapid evaporation of water results in lowering the tempera- 

 ture of the plant. The sunlight is rapidly absorbed by the chlorophyl 

 bodies, but only a small portion of this energy is transformed in the 

 making of carbohydrates. Much of the energy is passed through 

 the leaf, but a great deal of it becomes converted into heat. Under 

 conditions that interfere with transpiration, the temperature of leaves 

 exposed to sunshine increases so rapidly that the protoplasm is some- 

 times killed the leaves are actually scorched, although the tempera- 

 ture of the surrounding air may not be very high. This may be 

 observed in the summer time, when the sun comes out quickly after 

 a shower that has left a great deal of moisture in the air. The 

 moisture in the air prevents transpiration ; the sunshine is largely 

 converted into heat inside the leaves, and the protoplasm is injured 

 as a consequence. 



104. The guard cells. While there can be no doubt that the guard 

 cells are extremely sensitive to changes in outward conditions, it is 

 by no means certain just how their movements are related to the 

 work of the leaf, or whether, indeed, these movements have any 

 practical significance in the life of the plant. The movements of 

 the guard cells were formerly supposed to be related to the work 

 of transpiration. 



