Bitter Pit and Sensitivity to Poisons. 229 



COPPER SULPHATE. FIVE DAYS IN ONE LITRE OF SOLUTION, 

 TEMPERATURE AVERAGING 15°C. 



Strenffth of ( 1 per 1 per 1 per 1 per 1 per 



Solution ■( 100. 1000. 10,000. 100,000 1,000,000. 



Depth of 5-10 mm. 3-5 mm. 2-3 mm. Superficial brown- No signs of 



spots. iner only. poisoning. 



With stronger solutions of copper sulphate, when the potatoes 

 are first cut open, a bluish tinge can be seen beyond the browned 

 areas, but after exjjosure to air all this region browns, and it 

 consists wholly of dead cells. 



With solutions of lead nitrate, 1 per 10,000 strength, a faint 

 superficial browning appears on the prepared spots after five 

 days, but with mora dilute solutions (1 per 50.000), no signs of 

 poisoning can be detected. 



With strong solutions (1 per 1000 to 1 per 10), the tissues be- 

 neath the prepared spots leiuain white, and at first seem to show 

 no signs of poisoning. After a day's exposure to air, how- 

 ever, greyish, scabby spots 1-2 mm. deep develop with a 1 per 

 1000 solution, and this tissue is made up of dead cells. 



Potato juice produces a white flocculent precipitate with lead 

 nitrate, and this action seems to retard considerably the penetra- 

 tion of the lead nitrate. An excess of the latter apparently de- 

 stroys the oxidase ferment responsible for browning, and potato 

 slices immersed in 10 per cent, lead nitrate remain wdiite for an 

 indefinite length of time. A lesser strength of lead nitrate is 

 needed to kill the pulp cells than to destroy the oxidase. Hence 

 if potatoes are placed in a 5 per cent, solution, beneath each pre- 

 pared spot is an area of white but dead tissue, and then a brown 

 zone, in which the lead is less concentrated, and the oxidase has 

 had time to act after the cells were killed. Just in front of the 

 brown zone, a white line can be seen, where the cells have been 

 killed, but where no oxidation has as yet occurred, and further 

 in still tlie cells are living and normal. (See Plate xxiii.) To 

 produce browning, therefore, a time interval is necessary between 

 tho death of the cell and the destruction of the oxidase. 



With similar exposures at a temperature averaging 13-140 Q the 

 potatoes showed themselves insensitive to 1 per 100 solutions of 

 magnesium sulphate, and 1 per 2000 solutions of barium chlorate, 

 while only a faint superficial browning was produced by a 1 in 

 50 solution of magnesium sulphate, and a 1 in 500 solution of 

 Inarium chlorate. 



