LEAVES AND THEIR WORK 



127 



distributed to the air during twenty-four hours by a grass plot 

 twenty-five by one hundred feet, the size of the average city lot. 

 According to Ward, an oak tree may pass off two hundred and 

 twenty-six times its own weight in water during the season from 

 June to October. 



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 re- 

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

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



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



upper surface of one leaf and the lower surface of the other with vaseline. 

 The petioles of each should be covered with wax or vaseline, and the 

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

 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 weight. 

 Examination of the surface of a mullein leaf shows us that the lower sur- 

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

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



Regulation of Transpiration. — The stomata of leaves close at night. 

 On days when there is little humidity, they also tend to close, retarding 

 transpiration, but when the water supply is abundant they open, increas- 

 ing transpiration. This automatic action is of very great importance to 

 the life of a plant, since evaporation of water is thus regulated. 



The Effect of Transpiration on Water within the Stem. — It has al- 

 ready been noted that root pressure alone will not account for the rise 



