THE AIR 23 



This sap water, when it comes from the ground, is in a very dilute state; 

 and another function of the valves or mouths in the leaves, is to evaporate 

 water into the air, and in this way, to condense and concentrate the food 

 brought from the soil. This evaporation is also one of the means by which 

 the water is pumped up from below, and that there is a rapid evaporation 

 from the leaves is evidenced when we cut off a leafy branch and note how 

 quickly the leaves wilt when the supply is cut off. 



Wilting of the leaves is also one of nature's methods of protecting the 

 plant. During the hours of sunlight, and at no other time, the mouths in 

 the leaves are open to take in carbonic acid. But if the heat is great and the 

 soil is dry, the leaves will wilt, and this at once closes the pores or mouths, and 

 checks the evaporation until the plant can recover a supply from below. 



