314 ASSIMILATION. 



and exposed to different parts of a large spectrum formed by a 

 hollow prism filled with carbon bisulphide. By employing a nar- 

 rower slit for the light than that used b}' previous experimenters, 

 he obtained an exposure of the leaves to a very limited portion 

 of the spectrum ; and to this difference in his apparatus he chiefly 

 attributes his results, which are at variance with those of his 

 predecessors. Assuming that the results of his analysis of the 

 evolved gas are accurate, the}' indicate that the amount of car- 

 bonic acid decomposed by leaves is proportional to the distri- 

 bution of effective calorific energy in the spectrum. 



Timiriazeff' 1 in his earlier paper did not himself attempt to apply 

 his results to an explanation of the peculiar relations of the rays 

 of the spectrum to assimilation ; but Van Tieghem, who sub- 

 stantially adopts the results of Timiriazeff, gives the following 

 application of them to the associated phenomena. He calls atten- 

 tion to the fact that the maximum of decomposition of carbonic 

 acid, under the conditions of Timiriazeff's experiments, takes 

 place at the deep absorption-band of chlorophyll, between B 

 and C ; and therefore concludes that the decomposition of car- 

 bonic acid by leaves exposed to solar radiation depends on two 

 elements: (1) the elective absorption of the chlorophyll, and 

 (2) the calorific energ}~ of the absorbed radiations. According 

 to this view, the most efficient radiations must be those which, 

 being best absorbed by the chlorophyll, possess at the same time 

 the greatest calorific energy. Hence, (1) the extreme red and 

 the dark heat-rays, in spite of their extraordinary calorific energy, 

 have no effect, because they pass through chlorophyll without 

 visible absorption ; and (2) the blue rays, which are very strongly 

 absorbed, exert scarcely any effect, owing to their feeble calorific 



energy.' 2 



833. Timiriazeff's results should be compared with those of 

 Engelmann, who finds that for green cells the absolute maximum 







of assimilative activity lies in the red, between the lines B and C, 

 at the point of the first and most pronounced absorption-band of 

 chlorophyll, and that there is also more or less activity in the 

 blue at F. If the cells are not of a green color, the maximum of 

 activit\" is in some other point ; thus in the case of bluish-green 

 cells it is in the yellow, and in that of red cells in the green. 

 Engelmann's method is based upon the extraordinary sensi- 



1 Ann. cle Chimie et de Physique, ser. 5, tome xii., 1877, p. 394. and Ann. 

 des Sc. nat., ser. 7, tome ii., p. 99. 



2 Traite de Botanique, 1884, p. 149. 



