128 Carbon Assimilation. 



sucrose is a later formed product, and he supports this conclusion 

 with an analysis of leaf extract which shows only a negligible 

 quantity of sucrose in the leaf (about one-sixth of the quantity of 

 glucose) while the veins contain nearly five times as much sucrose 

 as hexoses. Strakosch's results are in direct contradiction to those 

 of the English workers, whose work we have summarised in the 

 preceding section of this chapter. Davis, Daish and Sawyer point 

 out that Grafe's test (1905) for fructose, used by Strakosch, is of 

 doubtful applicability in presence of glucose, while the method used 

 to localise glucose and sucrose, which is due to Senft (1904), is, 

 according to Mangham (1915), untrustworthy when sucrose is 

 present together with its hexose constituents. Davis, Daish and 

 Sawyer criticise Strakosch's results as well as Maugham's identifi- 

 cation of maltose in plant tissues, on the general ground that little 

 reliance can be placed on such microchemical tests as a means of 

 identifying one sugar in presence of others in plant tissues. The 

 fact that Mangham should claim to distinguish between rf-glucose 

 and rf-fructose in the plant by means of the osazone test, when their 

 phenyl osazones are of course identical, is not very reassuring as to 

 the degree of reliability of his results. 



Such qualitative observations as those of Strakosch and of 

 Dixon and Mason (1916) in any case cannot have the value of the 

 quantitative researches recorded in detail in the preceding section 

 of this chapter, and we find in all detailed quantitative examinations 

 made of the carbohydrates of the leaf, that cane sugar is always 

 present in the leaf in considerable quantity. We have already i 

 pointed out that all the workers who have obtained quantitative I 

 data have expressed the opinion tliat sucrose is the first sugar formed 

 in assimilation, and that this is inverted into hexoses for translocation. 

 When carbohydrate accumulates in the leaf in consequence of 

 assimilation taking place at a greater rate than translocation, the 

 excess of sucrose is supposed to be transformed into starch in 

 Tropceohwi (Brown and Morris). In the potato, which also forms 

 starch in the leaf, Davis and Sawyer (1916) appear to regard the 

 starch as formed from hexoses, soluble starch, which appears in 

 appreciable quantity in the leaf at the period when starch content is 

 at a maximum, being an intermediate product. This view is based 

 on the intimate relation between the content of hexoses and starch in 

 the leaf throughout the day, a relation, however, which is not very 

 obvious from Davis and Sawyer's published curves (cf. Fig. 17). 

 It must be admitted that the evidence in favour of the production of 



