Action, of ChemU'als on Fotato-Tiihers. 279 



The phenomenon is superficial, and its progress is governed 

 by gradual desiccation of the surface tissue, and also probably 

 by comparative lack of free oxygen in the subjacent tissue. 



The superficial layers to a depth of a few millimetres were 

 removed, and thin slices of the subjacent tissue in each series 

 were prepared. These were divided into two lots, one of which 

 was placed in Avater and heated nearly to boiling point and 

 maintained at this temperature for some minutes. The boiled 

 and unboiled sections were then exposed in a moist space, under 

 otherwise parallel conditions : after the lapse of 24 hours the 

 folloAving observations were made : — 



Boiled Unboiled Boiled Unboiled 



No visible Intense No visible Very intense 



oolor change coloration color change blackening 



The phenomenon is probably due to the action of the oxidis- 

 ing enzyme tyrosinase on the protein dissociation produce 

 tyrosin. The occurrence of tyrosin in potatoes has been de- 

 finitely established by StudeU, and by Schulze and Winterstein2. 

 The more intense darkening invariably observed in series B 

 and similarly treated material is probably due to the partial 

 disorganisation and death of the cellular units, and the conse- 

 quent freer diftusion of either the enzyme or tyrosin or both, 

 from cell to cell. 



Blight-fpee Intact Tubers. Sulphuric Acid Steeps. 



The outstanding fact furnished by the foregoing experiments, 

 viz., that even on prolonged steeping in brines ranging from 

 10 per cent, to 20 per cent, strength, the dissolved substance 

 only entered the steeped unpeeled tuber in small amount, led 

 to the institution of experiments with aqueous solutions of sul- 

 phuric acid. In each of the experiments summarised in the 

 tables beneath, five tubers were selected and separately steeped 

 in a 5 per cent, or 10 per cent, solution of sulphuric acid, for 

 the time periods indicated, and at laboratory temperature, 

 16° -18°Co. 



At the termination of each experiment the material was 

 removed, washed, divided into two lots of three and two tubers 



1 Deutsche med. Woch, 1900, p. 273. 



2 Zeitschr. f. physiol. Chem. 35, 299, (1902) and 45, 79, (1905). 



