Bittrr Pit. 349 



made over slightly bruised areas, which will subsequently give an 

 imitation of irregular pit formation. It was owing to the occa- 

 sional unreliability of the controls that I abandoned in my first 

 paper the use of pale-skinned, soft-fleshed apples. Even in sucli 

 ■cases, however, an element of doubt only creeps in in determining 

 the lowest limits of the poisonous concentrations. In the stronger 

 solutions every prepared spot shows a brown pit whicli has a well- 

 defined area centric to the prepared spot, and does not spread 

 beyond a sharp boundai'v zone. If when using a soft-fleshed or 

 pale-skinned apple, an occasional prepared spot on the control 

 develops an apparent " poison " pit; this is usually a pale colour. 

 If it is due to a bruise it will be irregular and not centric around 

 the spot. If it is due to too deep a cut admitting micro-organisms 

 it will slowly develop further in moist air, but the other prepared 

 spots will be unaffected. If it is due to an invasion of fungal 

 hypae, it will spread rapidly through the whole apple and the pulp 

 will become soft and watery. 



I might perhaps add that if red-skinned, hard-fleshed apples are 

 used, this method forms the best possible class experiment to 

 demonstrate — 



(a) the indifference of the plant-cell to distilled Avater ; 



(b) the importance of the cuticle; 



(c) the extreme sensitivity of the pulp cells to metallic poisons. 



For class experiments the best solutions to use are 1 gram per 

 100,000 of mercuric chloride or copper sulphate, and the method 

 can be used to some extent to test the freedom of distilled water from 

 small amounts of soluble metallic poisons. 



