132 Carbon Assimilation. 



of combustion have been made. Finally, we shall briefly deal with 

 work on the assimilatory power of light of differeht wave-lengths. 



B. Quantitative Estimation of Carbon Assimilation 



BY MEANS OF THE PRODUCTS. 



In an earlier chapter we have dealt with Blackman's and 

 Willstatter's estimation of carbon assimilation. Both these 

 workers employed a method based on that of Kreusler, in which 

 the intake of carbon dioxide by the leaf is used as a measure of 

 carbon assimilation.' The assimilation could also be measured by 

 estimating the increase of carbon content of the leaf, but what is 

 more usually done is to measure the increase of dry weight of the 

 leaf and assume that this is proportional to the increase in carbon 

 content. 



Brown and Escombe (1905) in their attempt to determine the 

 energy relations of the leaf, compared the increase of dry matter 

 with the intake of carbon dioxide, but as their results obtained by 

 the two methods were not concordant they came to the conclusion 

 that the dry weight method is untrustworthy. Therefoie they only 

 determined the intake of carbon dioxide and estimated the increase 

 in dry matter by calculation. We shall only deal briefly with the 

 extensive researches of Brown and Escombe on this subject, for 

 although they are the first to make quantitative measurements of 

 energy in regard to assimilation and although they clearly indicate 

 the complexity of the energy relations of the leaf, yet the values 

 actually determined by experiment are few and those obtained by 

 calculation are of doubtful value and do not agree with values 

 obtained by direct measurement by other investigators. We have 

 already referred to the divergence between the estimations of the 

 internal leaf temperature made by Brown and Escombe, and the 

 direct temperature measurements made by Blackman and Matthaei. 

 In order to estimate the dry matter formed. Brown and Escombe 

 multiply the weight of carbon dioxide absorbed by the leaf by a 

 carbohydrate factor of 0'640. This factor they obtain from the 

 analyses in regard to carbohydrate content of leaves of Tropceolum 

 tiiajus by Brown and Morris as described in the last chapter. Of 



' The same method has been employed by Brown and Escombe (1902). 

 We have not dealt with the very interesting i-esults recorded in this paper in 

 regard to carbon dioxide as a limiting factor, as the results are confirmed by 

 F. F. Blackman's later and more complete work on limiting factors. In order 

 to avoid unnecesary length of this review we have in this matter, as elsewhere, 

 confined our remarks to the more complete account where two researches run 

 parallel. 



