72 
THE GARDEN OF THE WORLD 
ture evaporates into the air just as the damp¬ 
ness evaporates from a wet rag that is hung on 
a clothesline. This process of giving off mois¬ 
ture is called transpiration. It is very like 
perspiration. It keeps the leaves cool, just as 
perspiration keeps human skins cool on a hot 
day in summer. 
Now, leaves, of course, cannot think and do 
not know anything about outside conditions. 
Their job is to bring up water for food making 
and get rid of what is left over. They are not 
able to know whether water is plentiful or 
scarce; the small pores open and pass off the 
moisture just the same. This is very hard on 
the plant during the dry season, for often it 
cannot get any moisture and the whole plant 
wilts. All the cells of the plant must be full of 
water, not just the leaf cells only. It is not at 
all unusual for delicate plants to die during a 
drought because the leaves transpire too much 
of the water the plant must have. 
Trees must be very careful of their water 
supply because they are so large that they use a 
