44 Ingvar Jorgensen and Walter Stiles. 
The conclusion they draw from their experiments is that blue 
and red light of the same intensity produce the same assimilation, 
Gr een 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 05 gm.-calories per sq. centimetre per minute 
could be reduced to ^ 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 0-0017 gm.-calories per sq. centimetre per minute 
i.e., 4-1% of the total incident energy. Now these workers found 
