ENERGY AND PHOTOSYNTHESIS 241 



the leaf. Their results, however, do not support Brown 

 and Escombe's figures, for the latter assumed that the 

 heat of combustion of all the photosynthetic products 

 might be comparable with that of glucose, which has been 

 shown not to be the case. 



An attempt to measure quantitatively the radiant 

 energy falling on a unit area of leaf surface in a unit of 

 time was made by Brown and Escombe in 1905, and to 

 determine what proportion of this energy was employed 

 in photosynthesis, how much in transpiration, and how 

 much was otherwise disposed of. That used up in 

 photosynthesis was, as we have seen, estimated by 

 measuring the quantity of carbon dioxide absorbed and 

 calculating the heat of combustion of the photosynthetic 

 products formed. The energy used in transpiration was 

 obtained by determining the amount of water transpired, 

 and the heat necessary to bring about the vaporisation 

 of that amount of water at a given temperature. These 

 two totals subtracted from the total incident energy 

 gave the amount lost in transmission, reflection, etc. 

 Without going into the details of Brown and Escombe's 

 experiments it may be said that they found that only a 

 surprisingly small proportion of the radiant energy was 

 used in photosynthesis, in one case only 0-6 per cent. 

 Puriewitsch obtained considerably higher values in some 

 cases, but neither Brown and Escombe nor Puriewitsch 

 appear to have taken into account the influence of 

 limiting factors, and thus their results cannot be regarded 

 as conclusive. 



Various attempts have also been made to estimate 

 the energy value of the different wave-lengths of the 

 spectrum. I have already stated to you the views of 

 earlier workers like Daubeny, Draper, Sachs, Pfeffer, 

 and Lommel as to the rays most efficacious in photo- 

 synthesis, and also the more detailed investigations 

 carried out by Reinke, Timiriazeff , and Engelmann, who 

 agreed in regarding the red rays absorbed by chlorophyll, 



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