78 HOW GREEN PLANTS MAKE FOOD 



their area. The amount of water lost by plants through evapo- 

 ration is many times more than the amount that goes into making 

 food and living matter. 



Experiment to show through which surface of a leaf water vapor passes off: 

 Remove two leaves of the same size from some large-leaved plant, as a mullein 

 or a rubber plant. Cover the upper surface of one leaf and the lower surface of 

 the other with vaseline. The leaf stalk of each should be covered with wax or vase- 

 line, and the two leaves exactly balanced on the pans of a balance placed in a warm 

 and sunny window. Within an hour the leaf having its upper surface covered with 

 vaseline will show a loss of weight. 



Factors in Transpiration. — The amount of water lost from a 

 plant varies greatly under different conditions. The humidity 

 of the air, its temperature, and the temperature of the plant all 

 affect the rate of transpiration. The stomata also tend to close 

 under some conditions, thus helping to prevent evaporation. 

 Recent experiments indicate that the plant probably has some con- 

 trol over the stomata. The stomata are usually closed at night 

 but remain open from shortly after sunrise until late in the 

 afternoon. They begin to close in the middle of the afternoon, and 

 thus decrease the amount of water lost in the latter part of the 

 day. Plants droop or wilt on hot dry days because they cannot 

 obtain water rapidly enough from the soil to make up for the loss 

 through the leaves. Hairs on the leaf surface, waterproofing of 

 outer cells, a decrease in leaf area, close grouping of leaves to 



