Jellett — On the Chemical Changes in Potato Disease. 737 



Table II. 





I. 



II. 



III. 



IV. 



Nitrogen (percentage in dry residue), 

 Sucrose (do.) 

 Glucose (do.) 



1-00 



•29 



1-59 



l-o3 

 1-27 



2-85 



l-o5 

 5-71 

 3-81 



1-26 

 1-00 

 1-93 



The history of these chemical changes seems to be as follows : — 



The first stage of the disease in the tuber is marked by an increase 

 in the quantity of nitrogen. 



This increase seems to have attained its greatest value before the 

 appearance of any discoloration in the tuber. 



The same stage of the disease is also marked by the development 

 ■of sugar, both glucose and sucrose. 



In the second stage, marked by a great increase in the discoloured 

 part of the tuber, the part which remains apparently sound shows no 

 increase of nitrogen but a very considerable increase in the quantity 

 of sugar. 



Finally, in the discoloured part of the tuber, there is a diminution 

 both in the percentage of nitrogen and in the percentage of sugar. 



Now it must be remembered, that in the vegetable kingdom the 

 fungi contain the largest percentage of nitrogen, approaching nearly 

 in this respect to the animal kingdom. A marked increase in the 

 quantity of nitrogen would therefore seem to indicate a fungoid growth 

 in the tuber. It would seem also that this growth attains a maximum 

 value before the tuber shows any visible sign of disease. 



The development of sugar appears to come somewhat later; at 

 least it continues for a considerable time after the percentage of nitro- 

 gen has attained its maximum value. There can be, I suppose, no 

 doubt that this sugar is formed by the conversion of the starch, which 

 the potato contains in large quantity. If the sugar produced were 

 wholly glucose, there would be no difficulty, as the presence of a 

 small quantity of acid would be sufficient to account for the pheno- 

 menon. But I am not aware that there is any known method by 

 which starch can be made to pass into sucrose. It is possible that 

 this effect may be produced by the presence of the fungus, which is 

 indicated by the increased quantity of nitrogen. I have not, however, 

 succeeded in establishing experimentally the possibility of this con- 

 version. 



The appearance of discoloration marks the commencement of decom- 

 position, and is attended, as we might naturally expect, by a diminution 

 in the quantity both of nitrogen and of sugar. 



3X2 



