Assiinilaiion in Relation to Radiant Energy. 151 



The conclusion they draw from their experiments is that blue 

 and red light of the same intensity produce the same assimilation. 

 Green light is incapable of producing assimilation. The absolute 

 intensity of energy incident on their plants is of the order of 0-005 

 gm. -calories per sq. centimetre per minute, and although it is likely 

 that with this low energy intensity, light is a limiting factor, yet 

 it cannot be assumed that this is so. Kniep and Minder appear to 

 be unaware of Blackman's work on limiting factors, and they give 

 no data relating to factors other than light intensity. It would, 

 therefore, be impossible to draw any valid conclusions from their 

 results, even if their experimental method were beyond criticism. 

 Investigations along another line have been made by attempting 

 to measure that part of the total energy absorbed by the leaf, which 

 is actually absorbed by the chlorophyll. The experiments of 

 Timiriazeff (1903), in which the absorption of radiant energy by 

 alcoholic extracts of leaves was taken as a measure of the absorption 

 of light by chlorophyll, are clearly of little value, as his leaf extracts 

 would contain far less chlorophyll than impurities. Also the state 

 of aggregation of chlorophyll and its distribution in the leaf are 

 different from those in an alcoholic extract. 



Again, the isolated experiments of Brown and Escombe (1905) 

 in which the absorption of radiant energy by the white and green 

 portions of a leaf of Negundo aceroides was compared and the 

 difference between the two values attributed to the chlorophyll, 

 is not to be regarded as providing any definite evidence, for it is 

 unfair to assume that the conditions in green and albino parts of a 

 leaf are identical except for the presence of the pigment, and 

 moreover, the considerations we have already put forward in regard 

 to Brown and Escombe's measurements of coefficients of absorption 

 hold equally well here. 



Nevertheless, in a comparatively recent publication, Weigert 

 (1911) has accepted Brown and Escombe's result for the Negundo 

 leaf, and applied it to work out the efficiency of the assimilatory 

 system for another species. Brown and Escombe had found that 

 in one of their experiments on the energy relations of the leaf, an 

 intensity radiation of 0'5 gm. -calories per sq. centimetre per minute 

 could be reduced to ^'j of this amount without diminishing the 

 rate of assimilation ; with further reduction of light intensity this 

 became the limiting factor. They estimated the energy used for 

 assimilation at 00017 gm. -calories per sq. centimetre per minute 

 «.«., 4-1% of the total incident energy. Now these workers found 



