XIII] PRINCIPAL FUNCTION OF THE LEAF 135 



a solution of chlorophyll so thin that it only absorbed the 

 red-orange rays between the lines B and C ; whereas they 

 flourished as well in the light transmitted by a solution 

 of potassium bichromate i.e. the red-orange in question 

 as in that transmitted by pure water. 



Hence we must conclude that although some of the 

 blue-violet rays are also absorbed, they are employed 

 chiefly for other purposes than assimilation : it is worth 

 noting, however, that the process of chloro- vaporisation 

 is most energetic in blue light, though it also occurs 

 in the red-orange. No doubt we may conclude that the 

 decomposition of carbon-dioxide is not the only work done 

 in the leaf by the energy absorbed as light. 



As to the quantity of light thus absorbed, it has been 

 shown that it is considerable, and it is probable that much 

 more energy is retained by the leaves than is necessary 

 for the actual assimilation of the carbon. 



Nevertheless the principal function of the leaves is 

 to extend the chlorophyll-layers in the light and air, as 

 said ; and the success with which they attain this object 

 is measured in various ways. 



As we have seen, the actively assimilating leaf evolves 

 large quantities of oxygen gas, the volume emitted being 

 equal to the volume of carbon -dioxide decomposed, and 

 much in excess of the small quantities of oxygen consumed 

 in respiration. 



The measurement of this gas-interchange has long 

 occupied the attention of physiologists, since its discovery 

 by De Saussure and others early in the last century ; and 

 the constancy of volumes of the carbon-dioxide absorbed 

 and decomposed, and of the oxygen evolved, supplies a 

 powerful argument in favour of our present theory of 

 amylogenesis. 



The volume of carbon-dioxide absorbed (and of oxygen 



