M. Wheldale 145 



Since no absolute reliance can be placed on the above tests, it is 

 only possible to draw deductions indirectly from such evidence as we 

 possess from other sources. 



Broadly speaking the concentration of sugars in a leaf depends 

 upon : 



(1) The assimilative activity. 



(2) The starch-forming activity. 



(3) The rate of translocation of sugars. 



Since these three factors are more or less intei'dependent and form 

 in co-operation a self-regulating mechanism, the concentration of sugar 

 as the outcome of their combined activities may under normal con- 

 ditions remain fairly constant. But if a tissue has assimilative without 

 starch-forming power or vice versa, we have perhaps some basis for 

 conjecture as to the concentration of its sugar-contents compared to 

 that of other tissues possessing both activities. There is a like possi- 

 bility if the different activities are affected in varying degrees by 

 changed conditions, and this question will be considered again in 

 connection with abnormal reddening in leaves. 



The question of the concentration of aromatic glucosides in a leaf 

 is even more problematic. Kraus(6) has given experimental evidence 

 for regarding the assimilating leaf as the seat of metabolism of aromatic 

 substances. This author found, as a result of analysis, that aromatic 

 compounds' accumulate in a cut leaf exposed to illumination but 

 decrease in a leaf kept in darkness. He moreover states the amount 

 of aromatic substances formed to be proportional to the assimilative 

 activity of the leaf 



Palladin(14) also holds the view that the aromatic materials of a 

 plant are manufactured from the carbohydrate series. In corroboration 

 of his view, he quotes the results of Waage(18), who obtained an increased 

 amount of phloroglucin in leaves fed on glucose solution ; also those of 

 Biisgen(2), who found that the tannin contents of plants increase in 

 glucose cultures. 



On such evidence as we have, we may say that the concentration of 

 aromatic substances in a leaf depends upon : — 



(1) The amount of sugars present in the leaf. 



(2) The rate of translocation of aromatic substances. 



' In this case, tanning, but the precise nature of the products is immaterial provided 

 they belong to the aromatic series. 



