LIGHT AND VEGETATION 93 



synthesis — the proportion of carbon dioxide in the air must be 

 increased. 



In experiments at the Smithsonian Institution in the 

 United States, the carbon dioxide content of the air in a glass 

 chamber was increased from 0-04/1000 (nine times less than 

 the normal concentration) to 5/1000 (fourteen times more 

 than the normal concentration) and the rate of photo- 

 synthesis of plants inside the glass chamber was measured. 



The experiment was made first with wheat in artificial 

 illumination of 4,000 lux supphed by incandescent lamps and 

 deprived of its excess of infra-red by a solution of copper 

 sulphate. This illumination is sixteen or seventeen times lower 

 than full sunhght, but to enable it to be used to the best 

 advantage it is necessary to increase the concentration of CO 2 

 in the air to 0-8/1000. With stronger illumination, about 

 10,000 lux, the optimum concentration would be 1-3/1000. 



Naturally, for the same illumination, it is useless to 

 increase still more the proportion of carbon dioxide in the air. 

 But extrapolation from the preceding results shows that 

 illumination of 70,000 to 80,000 lux, which is that of a fine 

 day at noon, would be fully used only if the proportion of 

 CO 2 in the air were 5/1000 to 6/1000; assimilation would then 

 be ten times more rapid than it is in normal air. 



At the same laboratory, an experiment was made on a 

 larger scale in daylight with wheat cultivated in a glass 

 chamber in which the atmosphere was enriched in carbon 

 dioxide by the slow circulation of the gas through it. The 

 concentration reached about 1-4/1000 — four times higher than 

 the normal. 



Compared with the control, the crop grown in the 

 artificial atmosphere produced more straw, more and larger 

 ears, and more seed. 



This example shows that in natural conditions the process 

 of photosynthesis, even in sun plants like cereals, is generally 

 Umited, not by an insufficiency of Ught but by other factors, 

 particularly by the poverty of the atmosphere in carbon 

 dioxide. 



