Ch. Ill, 6] TRANSPIRATION FROM PLANTS 



49 



the chlorenchyma from evaporating through them, and it 

 does so. The result is transpiration, which is thus primarily 

 not a function in itself, but an incidental accompaniment of 

 the food-forming process. The formation of a given amount 

 of food requires a definite amount of carbon dioxide, and 

 this means so much open stoma, and therefore loss of water, 

 in definite mathematical 

 proportions. 



The stomata are slit- 

 like openings which de- 

 velop by separation of 

 the walls of the young 

 epidermal cells. In so 

 far as the passage of 

 gases is concerned, they 

 might to advantage re- 

 main permanently open ; 

 but in fact they open and 

 close, with a proportion- 

 ate effect upon transpir- 

 ation. The opening and 

 closing in each case is 

 produced by action of 

 two neighboring epider- 

 mal cells, specialized as 



GUARD CELLS (Fig. 22), 



or which the walls are so 

 thickened as naturally to spring the cells together, thus clos- 

 ing the stoma; but the absorption of more water into the 

 sap-cavities rounds out the cells and draws them apart, thus 

 opening the stoma to a slit, a spindle form, or even, at an 

 extreme, to an almost circular opening. Thus the mechanism 

 is such that when the cells of the leaf are collectively losing 

 water faster than it is restored from the stem, the guard cells 

 tend automatically to close the stoma, checking proportion- 

 ally the transpiration, while the access of more water to the 



Fig. 22. — A typical stoma, with guard 

 cells, of Thymus, seen from the surface, 

 and in cross section. The operation of the 

 guard cells is explained in the text. (After 

 a wall-chart by L. Kny.) 



