MANUFACTURE AND UTILIZATION OF CARBOHYDRATES 163 



Table 58. — The Relation of Carbon Dioxide Assimilated to the Number of 



Stomata in Leaves Illuminated on the Upper Surface 



{After Brown and Escomhe--) 



than might be expected from the number of stomata on the upper side. 

 This is because the leaves were ilhmiinated from above, resulting prob- 

 ably in a greater degree of opening of the stomata and a more rapid 

 utilization of carbon dioxide by this side of the leaf. Both of these 

 factors would favor a more rapid intake of carbon dioxide. 



The large amount absorbed by a leaf during active assimilation 

 despite the low partial pressure of carbon dioxide in the atmosphere and 

 the small fraction of the leaf surface occupied by stomata is explained 

 by Brown and Escombe's law"^ which states that diffusion through a 

 perforated membrane is proportional to the diameter of the apertures and 

 not to their area. Because of the small size of the stomata, their great 

 number and their distribution over the surface, the amount of carbon 

 dioxide that theoretically could be taken in by the leaf under the most 

 favorable circumstances is much greater than any observed quantity 

 absorbed. Some idea of the amount used by leaves is given by an experi- 

 ment of Brown and Escombe^^ on the sunflower, in which they found 

 that approximately half a liter of carbon dioxide was used by each square 

 meter of leaf surface in an hour. 



The carbon dioxide content of the atmosphere is constant; therefore 

 it is not a factor to be considered in fruit growing. However, when it 

 is artificially changed, in the absence of other limiting factors, the rate 

 of assimilation increases in proportion to an increase in the carbon dioxide 

 supply until an atmospheric concentration of 30 to 50 per cent, is reached. 

 Cummings and Jones^" have obtained very marked results from aerial 

 fertilization with carbon dioxide. Legumes fertilized in this way showed 

 increased carbohydrate storage and an increased production of pods 

 and beans. Potatoes produced better tubers and strawberries showed 

 distinct effects. This probably holds until an atmospheric concentration 

 of about 30 per cent, or more is reached. Atmospheric concentrations 



