LEAVES AND THEIR USES 243 



part of the living matter itself as well as nearly all of the cell 

 sap, and too great a loss of water stops the activities of living 

 matter and finally kills it. This is what goes on in both 

 wild and cultivated plants in a dry summer. Even large 

 trees sometimes die after a series of dry seasons. 



Much of the evaporation from leaves, although it is use- 

 less and even dangerous to the plant, is unavoidable, because 

 leaves must expose a large surface to the air in order to 

 absorb enough carbon dioxid, and also in order to receive 

 sufficient light for carbohydrate manufacture. The exposure 

 of a large leaf surface for these purposes means that much 

 water will be lost by evaporation ; and so the needs of the 

 leaf for light and carbon dioxid seem to explain why the 

 plant must do so much apparently unnecessary work in 

 pumping up water from the soil. 



258. How Evaporation is Checked. — So serious is the 

 danger of too great a loss of water that many peculiarities 

 in the structure of leaves are to be explained by the need of 

 keeping the rate of evaporation as low as possible. Chief 

 among the devices for checking evaporation are the minute 

 air-pores which are present in great numbers on the surfaces 

 of leaves (see Fig. 52). The pores lead into intercellular 

 spaces within the leaves. Experiment has shown that the 

 gases of the air (including carbon dioxid) pass into and out 

 of these pores and come into contact with the inner cells of 

 the leaf about as readily as though these cells were not 

 covered by the epidermis, while the evaporation of water 

 from the inner cells is slower than it would be if the epidermis 

 were not there. The practical effect, then, of the presence 

 of air-pores and intercellular spaces is that the area of leaf- 

 cells which can absorb carbon dioxid is greatly increased 

 without a corresponding increase in the amount of water 

 evaporated. This result is helped also by the fact that as a 

 rule the air-pores are found (especially in plants living in 

 dry places) on the lower surface of the leaf, which is shaded 



