22 THE SYNTHESIS OF CARBOHYDRATES 



is so enriched with carbon dioxide that light and not carbon 

 dioxide is limiting the rate, that a regular rate of carbon 

 assimilation is maintained for over twenty-four hours in a 

 constant illumination. Maskell,* to whom these observations 

 are due, suggests that the apparent rate of carbon assimilation, 

 when carbon dioxide is limiting, is determined by the external 

 carbon dioxide concentration and a series of resistances which 

 comprise the stomatal resistance to diffusion, the resistance 

 in the intercellular space system and in the liquid diffusion 

 path from the cell surface to the surface of the chloroplast 

 together with the resistances associated with the photochemical 

 and dark phases of the actual process. 



In natural conditions carbon assimilation is limited by the 

 low pressure of carbon dioxide in the atmosphere, the high 

 values obtained in experiments in conditions involving an 

 increased supply of the gas never being attained. 



In experimental work with land plants a limit is set to the 

 increase of carbon dioxide supply by the narcotic effect of the 

 gas when in excess, f 25 per cent, generally will inhibit growth ; 

 aquatic plants, on the other hand, are able to withstand a 

 relatively high concentration. Blackman and Smith J found 

 that Fontinalis and Elodea in water, with a carbon dioxide 

 concentration of 33-92 and 35-82 per cent, of saturation, and 

 under identical intensity of illumination and at temperatures 

 of 23 C. and 28 C. respectively, assimilated -0223 and -0249 

 gram of carbon dioxide per hour per standard area of 137 

 sq. cm. Fontinalis is less efficient, for reasons not finally deter- 

 mined, than aquatic angiosperms such as Elodea, Potamogeton 

 and Ceratophyllum in utilizing carbon dioxide. Blackman and 

 Smith, experimenting with Elodea and Fontinalis in constant 

 conditions of light and temperature and with a carbon dioxide 

 supply ranging from -0025 to -0540 gram per 100 c.c. of water, 

 found that the carbon assimilation increases steadily in pro- 

 portion to the increase in the supply of carbon dioxide. When 

 tin- assimilation reaches about -023 gram of carbon dioxide 



* Maskell : " Proc. Roy. Soc," B, 1928, 102, 467, 488. 



f See Chapin : " Flora," 1902, 91, 348. 



% Blackmail and Smith : " Proc. Roy. Soc," B, 1911, 83, 389. 



