3T4 The Physiology of Plants bookTii 



sponding to the band in the red between B and C, which he 

 said possess a high mechanical intensity. He is thus held 

 to have enunciated the idea that the reduction of the carbon 

 dioxide may be considered a function of the energy of 

 radiation and of the degree of its absorption by chlorophyll. 



It must be noticed, however, that as Timiriazeff pointed 

 out, Herschel's law does not quite apply in this case, 

 as the rays are absorbed by the chlorophyll, while the 

 chemical change is concerned with the carbon dioxide 

 which does not absorb the light at all. Timiriazeff suggested 

 that the function of the chlorophyll is really to act as 

 a sensitizer, to absorb the rays, that is, and thus to enable 

 them to do chemical work outside the actual absorbent. 

 With its action in this particular he associated the fact of 

 the existence of the two related pigments, chlorophyll and 

 protophyllin in the plastid, allusion to which has already 

 been made in another connexion. 



Lommel's view, that the rays absorbed by the pigment 

 are the effective rays in photosynthesis, which seems to 

 have been accepted at once by other workers, was the 

 starting-point for most of the researches to which attention 

 must now be directed. 



Mention, however, must first be made of an alternative 

 hypothesis brought forward by Pringsheim in 1874. He 

 urged that the rays absorbed are not those concerned in 

 the decomposition of carbon dioxide, but that they are cut 

 out because they would be deleterious to the constructive 

 mechanism. Chlorophyll, on this hypothesis, is nothing more 

 than a filtering mechanism, sifting the rays of light, and 

 keeping from the plant those which would interfere with 

 its constructive work. This view excited a certain amount 

 of comment when it was put forward, but it was not based 

 upon experiment, and soon fell into the background. It 

 was finally disproved by the observation of Nagamatz in 

 1886 that light which has passed through a leaf has lost its 



