PLANTS MAKE FOOD 



87 



From which Surface of the Leaf is Water Lost ? In order to find out 

 whether water is passed out from any particular part of the leaf, we may 

 remove two leaves of the same size and weight from some large-leaved 

 plant l a mullein was used for the illustrations given below and cover 

 the upper surface of one leaf and the lower surface of the other with vase- 

 line. The leaf stalks of each should be covered with wax or vaseline, and 

 the two leaves exactly balanced on the pans of a balance which has pre- 

 viously been placed in a warm and sunny place. Within an hour the leaf 

 which has the upper surface covered with vaseline will show a loss of 



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



weight. Examination of the surface of a mullein leaf shows us that the 

 lower surface of the leaf is provided with stomata. It is through these organs, 

 then, that water is passed out from the tissues of the leaf. 



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. But 

 there seems to be no certain regulation of this water loss. Conse- 

 quently plants droop or wilt on hot dry days because they cannot 



1 The "rubber plant" leaf is an easily obtainable and excellent demonstration. 



