ABSORPTION OF CARBON 



35 



method already described, the blue-color reaction will be observed 

 only in those portions where the stomata remained open (Fig. 12). 



It is well known that the degree of opening of the stomata is 

 not always the same. They are able to widen and to narrow their 

 apertures or even close them entirely. These changes in the 

 degree of opening have an influence on the process of assimilation. 

 The closing of stomata stops it entirely. Therefore, environ- 

 mental conditions that induce the closing of the stomata, usually 

 lack of moisture, also check photosynthesis. During periods of 

 drought plants actually suffer from starva- 

 tion, they cease to develop and even lose in 

 weight. This explains the poor develop- 

 ment of plants in dry regions, as well as the 

 low yields in years of drought. Assimilation 

 is likewise suspended in evergreen plants 

 during the cold season. Though they pre- 

 serve their assimilating leaves, these organs 

 are inactive, since in winter the stomata are 

 permanently closed. 



Even in the fully opened condition the 

 stomata occupy but a small part of the leaf 

 surface, not more than 1 per cent of the whole 

 area of the leaf. One would think that the 

 diffusion of carbon dioxide into the leaf must 

 be greatly impeded, as the gas has to pass 

 through very small openings. Brown and FlG - 12 - — Influence of 



-,-,,, 11, , , coating of stomata on 



.bscombe, however, were able to prove that the formation of starch 

 an epidermis, perforated by numerous small in the leaf (f rom Paiia- 

 openings, is less of an obstacle to the diffusion 

 of carbon dioxide than might be supposed. Thus, in one of the 

 experiments conducted with the leaf of Catalpa, it was observed 

 that 1 sq. cm. of the leaf's surface absorbed 0.07 cc. of carbon 

 dioxide per hour. An equal surface of an alkali solution absorbs 

 during the same period from 0.12 to 0.15 cc. of CO2, or only twice 

 more. It must be noted, however, that of 1 sq. cm. of leaf surface 

 only 1 sq. mm., or one one-hundredth of its area, is occupied by the 

 stomatal openings, the remaining 99 per cent being impermeable 

 cuticle. It may be calculated from this that the carbon dioxide 

 penetrates into the stomata at a rate fifty times higher than the 

 rate in entering a solution of alkali. 



