50 A TEXTBOOK OF BOTANY [Ch. Ill, 6 



leaf, permitting renewed turgescence of the guard cells, pro- 

 duces a reopening of the stoma. One other important con- 

 dition, however, influences this result. The guard cells, alone 

 of the epidermal cells, contain chlorophyll, and hence make 

 grape sugar in light ; and a solution of grape sugar, as will 

 later be shown, draws water osmotically from neighboring 

 cells, thus increasing the turgescence of the guard cells and 

 opening the stoma. Accordingly, while the stomata tend to 

 close with dryness, so to speak, they also tend to open in 

 light, which is the time when carbon dioxide is needed in the 

 work of the leaf. These two conditions, however, often oper- 

 ate antagonistically, producing irregularities in the action of 

 the guard cells. Thus, while their operation can be viewed 

 as adaptive in general, it is not so in detail. In this respect 

 the stomatal mechanism resembles most other adaptations, 

 which, because so many other factors are simultaneously 

 affecting the part concerned, can never be perfect. 



Stomata occur chiefly, and in most plants exclusively, 

 on the under sides of leaves, in which position a stoppage 

 of their openings, and therefore of gas passage, cannot be 

 caused by rain. Against this detriment several adaptations 

 have been described, though often misinterpreted as a sup- 

 posed need for promoting transpiration. Stomata vary 

 much in size, extent of opening, and number, ranging from 

 all the way up to near 500 per square millimeter. Their 

 conventional constant (page 25) is 100 per square millimeter 

 of surface, and their area when extended the widest possible 

 would open -^ of the leaf surface (Fig. 23). It is at first 

 puzzling to the observer, as it long was to botanists, how, 

 through so small a total area of opening, a sufficiency of 

 carbon dioxide can enter and so much water vapor escape. 

 The explanation has been found in a very curious physical 

 fact, viz., that the smaller an opening becomes, the more rapid 

 relatively (not absolutely) is the passage of a gas through it 

 by diffusion, while such passage is also more rapid through 

 slit-shaped than through round openings of the same area. 



