24 THE SYNTHESIS OF CARBOHYDRATES 



only the free carbon dioxide is available for carbon assimi- 

 lation ; such a solution when dilute— the range used by James 

 varied between -05 to 0-4 per cent. — is suitable for experimental 

 work. Stronger solutions are to be avoided owing to the 

 injurious effect of the increased alkalinity. 



The difficulties inseparable from critical investigations in 

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

 the carbon dioxide absorption of Cocos leaves in natural 

 conditions may be mentioned. He found that the rate of 

 absorption is at a maximum in the morning, a depression ob- 

 tains 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 nor- 

 mally obtain with attached leaves, for which difference there 

 is no adequate explanation. Clearly some internal limiting 

 factor is operating, possibly connected with the accumulation 

 of the products of carbon assimilation (p. 49). It was also 

 found that immature and old leaves absorbed carbon dioxide 

 at a lesser rate than leaves of an intermediate age. 



In view of the small amount of carbon dioxide in the normal 

 atmosphere, it must not infrequently happen that the inten- 

 sity of carbon assimilation is limited by this factor, wherefore, 

 since the days of Kreusler,f who was the first to consider 

 this aspect, much work has been done both on the Continent 

 and, more recently, in this country to increase the crop yield 

 by the enrichment of the atmosphere by the addition of 

 carbon dioxide supplied by artificial means. $ The results 

 obtained are in some instances contradictory, presumably 

 owing to the difficulty of control in field cultivation, and also 

 in greenhouse culture where it is difficult to maintain a con- 

 stant increased pressure of carbon dioxide although tempera- 

 ture, humidity and soil conditions are amenable to regulation. 



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



t Kreusler : " Landw. Jahrb.," 1885, 14, 913. 



X Ccrighclli : " Ann. Sci. Agron.," 1921, 38, 68. Demoussy : " Compt. 

 rend.," 1903, 136, 325 ; 1904, 138, 291 ; 1904, 139, 883. Fischer: " Beih. 

 deut. bot. Ges.," 1927, 45, 331. Lundegardh : " Der Kreislauf der 

 Kohlensaure in der Natur," Jena, 1924. Reinau : " Zeit. angew. Chem.," 

 1926, 16, 495. Reidel : " Chem. Zeit.," 1921, 104, 829. 



