240 HISTORY OF BOTANY 



value in determining the presence of one sugar when 

 others are present also. To sum up, in the words of 

 Jorgensen and Stiles, " While we may regard starch as 

 a secondary product of assimilation, and while also there 

 is good evidence that carbohydrates are translocated, 

 as hexose sugars in some cases, or to some extent at any 

 rate, and while there is strong evidence that sugars are 

 the first definitely known products of the assimilatory 

 process, there is no evidence at present as to which 

 particular sugar is the first one to be produced in the leaf." 



ENERGY AND PHOTOSYNTHESIS 



During the entire period that a leaf is exposed to 

 light, radiant energy is falling on its surface ; part of that 

 energy is transmitted through the leaf and part is absorbed. 

 What becomes of the energy absorbed, and how can its 

 amount be estimated ? Kreusler, in the later years 

 of the nineteenth century, calculated the intensity of 

 photosynthesis by measuring the amount of carbon 

 dioxide absorbed by the leaf. In 1905 Brown and 

 Escombe also employed this method, comparing the 

 results they obtained with the observed increase in dry 

 weight. Finding that these results did not correspond, 

 they confined themselves to the first of the two methods 

 and calculated the dry weight from the observed C0 2 

 intake, the increase in dry weight being taken as pro- 

 portional to the amount of carbohydrate formed. Some 

 doubt was thrown on the reliability of Brown and 

 Escombe's results by Thoday in 1909, who found con- 

 siderable variations in the proportions obtaining between 

 the increase in dry weight and increase in carbon content 

 in the leaf. 



Several investigators (notably Puriewitsch, in 1914) 

 have in recent years attempted to estimate the heat of 

 combustion of photosynthetic products, with the object 

 of gaining some knowledge of the energy relations of 



