216 PHOTOSYNTHESIS 



gists in endeavoring to explain the synthesis of glucose and the further 

 synthesis of starch. 



The first to subject the problem as to what is the first sugar formed 

 to quantitative experimental investigation w^ere Brown and Morris.^^ On 

 the basis of chemical analyses of leaves they came to the conclusion that, 

 not glucose, but sucrose is the first sugar formed in photosynthesis. Their 

 argument is based upon analyses of Tropaeolum (already quoted). These 

 analyses show a decided increase in the amounts of sucrose and fructose 

 after the leaves have been illuminated. There is no appreciable increase 

 in the amount of glucose. The accumulation of fructose is explained on 

 the basis that this sugar results from the inversion of sucrose ; the equiva- 

 lent quantity of glucose, which is simultaneously produced in the inversion 

 is used up in the process of respiration. Separate experiments of Brown 

 and Morris would go to show that in the dark there is a decided loss of 

 sucrose and that glucose is more rapidly used in respiration than fructose, 

 so that there is an accumulation of fructose. According to this view 

 sucrose is the first sugar formed and serves as a temporary reserve 

 material ; when the concentration exceeds a certain limit starch commences 

 to be formed. The fact, observed by Brown and Morris, that, although 

 the leaves had been illuminated for a long period and the amount of 

 glucose did not increase, is taken as evidence that glucose is not the 

 first sugar formed. 



The great weakness in the experiments of Brown and Morris is the 

 fact that the determinations of glucose and fructose are of very doubtful 

 value. It is extremely difficult to determine these two sugars accurately 

 when they are in a mixture such as is obtained from a leaf extract and, 

 as Davis ^°° has shown, small admixtures of non-carbohydrates, showing 

 optical activity, can introduce very great errors. Also, as we know now, 

 the system studied by Brown and Morris, containing glucose, fructose, 

 sucrose, maltose and starch is too complex to permit accurate results to 



be obtained. 



A simpler condition was presented by the snowdrop ( Galanthus nivalis, 

 L.), which was studied by Parkin.^"^ The leaves of this plant contain 

 only glucose, fructose, and sucrose. Parkin also favors the view that 

 sucrose is the first sugar formed. He concludes: "During any single 

 day of the spring the percentage of hexose sugars in the leaf remains fairly 

 constant, no matter at what hour of the twenty-four the leaves may be 

 examined. That of the sucrose, on the other hand, fluctuates greatly. It 

 increases during the day and diminishes during the night. Further, leaves 

 detached and insolated contain decidedly more sucrose than their controls, 

 but the quantity of hexose sugars remains much the same." 



With improved methods of analysis Davis "' and his collaborators have 



•"Brown and Morris, Jour. chem. Soc, 63, 604 (1893). 



""Davis, Jour. Agri. Sci., 7, 327 (1916). 



io»p„_i.:r, Binchem Jour., 6, 1 (1911). 



-Daviwf at W. Agr. Sci., 5, 437 (1913) ; 6, 152, 406 (1914) ; 7, 255, 327, 



352 (1916)'. 



