208 PHOTOSYNTHESIS 



hydrates, due regard must be given the transformations which these com- 

 pounds undergo quite independently of the photosynthetic process. 



The quantitative determination of starch in leaves has usually been 

 carried out by first hydrolyzing the starch by means of an enzyme and 

 then determining the resulting reducing sugars v^ith a cuproalkaline solu- 

 tion. The method of O'SuUivan,''' which makes use of diastase for the 

 hydrolysis of starch and which was used by Brown and Morris has the 

 disadvantage that the dextrin which is formed is carried down by the 

 subsequent treatment with lead acetate and the determinations are conse- 

 quently low. Davis and Daish "' used takadiastase which converts the 

 starch entirely into maltose and glucose ; these are then determined by 

 measurement of the copper reducing power and optical rotation. In the 

 absence of fructose and sucrose, the maltose and glucose mixture resulting 

 from the action of the takadiastase on starch, can also be treated with I.N. 

 solution of hydrochloric acid which converts all the maltose to glucose 

 and the latter can then be determined by its reducing power. ^^ 



Inulin. An analogous position to starch in some plants is taken by 

 inulin. These plants instead of storing starch accumulate inulin. On 

 account of the fact that inulin is much more soluble in water than starch 

 the circumstances eflfecting the accumulation of the former in different 

 parts of the plant are different from that of starch. Inulin is usually found 

 in the storage organs, but also occurs according to Grafe and Vouk ''^ in 

 the leaves of Cichorium intybus. These authors report a considerable in- 

 crease of inulin in the leaves during photosynthesis and consider that inulin 

 is directly produced in the photosynthetic process. Colin, on the other 

 hand, considers that the inulin is not present in the leaves, but is condensed 

 in the stem, 



. 3. The Transformations of Carbohydrates in the Leaf 



It is not our intention to undertake here an exhaustive discussion of 

 the transformations which the carbohydrates undergo in the leaf. We 

 wish merely to show the necessity of considering these transformations in 

 attempting to arrive at any logical conclusions regarding the nature of 

 the carbohydrates formed in the photosynthetic process. It will become 

 evident that the various carbohydrates undergo changes quite independently 

 of the photosynthetic process and hence due regard must be given the 

 conditions effecting these changes in experiments designed to establish the 

 nature of the photosynthetic products. These conditions include light 

 intensity, duration of illumination, temperature, water-content, rate of 

 respiration and the previous history of the plant. As a consequence of 

 differences in these conditions the nature of the photosynthetic products 



'« O'Sullivan, Jour. Chem. Soc, 45, 1 (1884). 

 "Davis and Daish, Jour. Agri. Sci., 6, 152 (1914). 

 "Tollenaar, Dissertation, Wageningen, 1925. 



™Grafe and Vouk, Biochcm. Zcif., 43. 424; 47, 320 (1912); 56, 249 (1913). 

 Colin, Rev. Ccn. Bat., 31, 70, 179, 229, 277 (1919) ; C. A., 14, 1134 (1920). 



