1917] CURRENT LITERATURE 519 
power of light of different wave lengths. Under (1) two methods of estimating 
the products of carbon are discussed: (a) increase in dry weight, and () amount 
of CO. taken in. The reviewers conclude that if the dry weight method can 
be made more accurate, it should not be lightly abandoned. If we assume with 
BRowNn and Escomse that the heat of combustion of all products of assimilation, 
is the same as that of glucose (3.76103 gram calories), we shall fall into a 
considerable error, as is indicated by the following values for other substances 
present in leaves: sucrose 3.99103, starch 4.110%, cellulose 4.210%. If 
oils are present, the error in this assumption would be still greater. Actual 
determinations of heat of combustion made by other workers on the products 
of assimilation in the leaves of various plants give values varying from 4.4 103 
to 5.2X103 gram calories. 
Quantitative measurements of radiant energy in relation to the leaf are 
based on the assumption that the total radiant energy falling upon the leaf is 
disposed of in the following ways: (1) reflection from the leaf surface, (2) carbon 
Le (3) transpiration, (4) transmission through the leaf, (5) thermal 
epee and EscoMBE (1905) disregard (1) in their calculations. The 
reviewers believe that this is not negligible, since even a black cloth may reflect 
I per cent of the radiant energy incident upon it. PwuRIEWwITscH (1914) has 
estimated (2) in a few cases by measurement of the increased heat of combus- 
tion of the leaf per unit area. His highest value was 2.6 and his lowest 1.3 per 
cent. On the basis of these he calculated other cases, getting as high as 7.7 
per cent. Brown and Escomse calculated (2) by assuming that one gram of 
absorbed CO, is equivalent to 0.64 gram of dry matter formed, and that the 
heat of combustion of the products of assimilation is 3.76103 gram calories, 
The accuracy of these assumptions is not confirmed by measurements. Their 
computed values vary from 0.42 to 1.66 per cent. All of the evidence at 
hand thus indicates that only a very small percentage of the radiant energy 
received by the leaf is actually used in carbon assimilation. It might be 
expected that the proportion of the sun’s energy used in assimilation would 
vary inversely as the intensity of the illumination. This expectation is not 
justified by the experimental data, and it is clear that we must look for some 
other factor on which no data are given. The reviewers point out here a 
case of lack of correlation of effort by investigators. If Purrewrrscu had 
taken cognizance of BLAcKMAN’s researches, his experiments (although 
regarded by Purrewitscu himself as preliminary) might have yielded results of 
much greater significance. The energy used in (3) was arrived at by Brown 
and Escomse by determining by weight the amount of transpiration and 
calculating the energy used from the heat of the vaporization of water at that 
reviewers vary but little, the highest being 35.32 and the lowest 35.28 per 
cent. It is evident that (5) will usually have a positive value, since the 
