92 LIGHT, VEGETATION AND CHLOROPHYLL 



light? In laboratory experiments, its activity is generally 

 measured either by the absorption of carbon dioxide or by 

 the liberation of oxygen. The action of light is known to 

 depend on other factors, such as the carbon dioxide content 

 of the air and the temperature, and this dependence is 

 governed by complex rules. 



The simplest and most useful indication in these phenomena 

 is the law of the minimum, according to which the lowest 

 factor regulates the rate of assimilation. 



Suppose, for example, that the light is poor but that the 

 temperature is favourable and the atmosphere is artificially 

 enriched with carbon dioxide. The plant will have an 

 abundance of everything except Ught. It is Hght, therefore, 

 which will check the photosynthesis and regulate its speed. 



In these conditions, and within a limited range of variation, 

 the rate of photosynthesis is proportional to the illumination. 

 Then, as the light becomes more abundant, the rate increases 

 more slowly and is finally stabHlized. A further increase of 

 illumination is useless because the rate is checked by another 

 factor. 



In ordinary conditions, and for the majority of plants, 

 photosynthesis reaches its maximum with relatively low 

 illuminations; for wheat, for example, the value is ten times 

 lower than that of full sunhght. That is why crops can be 

 grown with record rapidity in the summer in high latitudes 

 with very long days, in spite of the considerable atmospheric 

 absorption due to the sun remaining low on the horizon; 

 photosynthesis takes place at the same rate as it does in districts 

 where the Hght is stronger, but it continues without inter- 

 ruption, while in lower latitudes the nights are longer. 



Adaptation also plays an important part; a plant raised 

 in poor light makes much better use of it than a plant 

 raised in full daylight. 



The Influence of the Pressure of CO 2 



When the illumination is adequate, it is therefore necessary 

 to control another factor which Umits the rate of photo- 



