22 THE SYNTHESIS OF CARBOHYDRATES 



low pressure of carbon dioxide in the atmosphere, the high 

 values obtained in experiments under 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,* 25 per cent generally will inhibit growth ; 

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

 relatively high concentration. Blackman and Smith t 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 

 grams of carbon dioxide per hour per standard area of 137 sq. 

 cm. Fontinalis is less efficient, for reasons not finally determined, 

 than aquatic angiosperms such as Elode.i, Potamogeton and 

 Ceratophyllum in utilizing carbon dioxide. Blackman and 

 Smith, experimenting with Elodea and Fontinalis under 

 constant conditions of light and temperature and with a carbon 

 dioxide supply ranging from -0025 to '0540 gms. per 100 c.c. 

 of water, found that the carbon assimilation increases steadily 

 in proportion to the increase in the supply of carbon dioxide. 

 When the assimilation reaches about -023 grams of carbon 

 dioxide per hour, however, there is no further increase with 

 an augmented carbon dioxide unless the light intensity be 

 increased. In other words, light intensity in this connection 

 is a limiting factor. 



The difficulties inseparable from critical investigations in 

 the field are obvious : of recent work, that of McLean J on the 

 carbon dioxide absorption of coconut leaves under natural 

 conditions may be mentioned. He found that the rate of 

 absorption is at a maximum in the morning, a depression 

 obtains at mid-day followed by an increase in the afternoon 

 and then a final decline towards sunset. Similar values were 

 obtained for detached leaves, but the curve showed a single 

 maximum at about noon instead of two maxima which 

 normally obtain with attached leaves, for which difference 

 there is no adequate explanation. Clearly some internal 



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



f Blackman and Smith: " Proc. Roy. Soc." Lond. B. 1911, 83, 389. 



I McLean: "Ann. Bot." 1920, 34, 367. 



