Leaves. 



95 



stretch and spread apart; but in the nighttime the chlo- 

 roplasts of the guard cell no longer manufacture new 

 material ; the cells accordingly lose their turgidity and are 

 drawn together again by the elasticity of their walls (see 

 Fig. 40). 



When the soil is dry, and 

 the amount of water rising 

 from the roots is much 

 reduced, the guard cells 

 probably lose water faster 

 than they are able to draw 

 it from surrounding tissues, 

 and so are incapable of ac- 

 quiring the degree of tur- 

 gidity necessary to their 

 opening. From such simple 

 causes as these, the plant 

 may allow the ingress of 

 carbon dioxide when the 

 conditions are such that it 

 can be employed in food 

 making, and may guard 

 against too great loss of water when the supply of it is 

 scarce. 



73. The World's Food Supply. When we consider pho- 

 tosynthesis from the standpoint of the world's food supply, 

 it becomes a subject of supreme interest. The entire 

 substance of seeds, tubers, bulbs, and roots, which directly 

 or indirectly constitute the food of mankind, was only 

 a few months past scattered in -the form of water, soil 

 particles, and gases of themmosphere, no amounts of which 

 could keep the world from starvation ; and the continuance 

 of animal life has be&n made possible only through the 



FIG. 40. 



a, surface view of an open stoma; , 

 surface view of a closed stoma (after 

 HANSEN) ; c, diagram of a transverse 

 section of a stoma. The light lines in- 

 dicate the closed position of the guard 

 cells and the heavy line the open posi- 

 tion (after SCHWENDENER). 



