282 DISEASE IN PLANTS. 



own roots would have done, and probably mingles 

 with them a certain amount of its own peculiar 

 products, as well as affects the modes of working 

 and interaction of both by the molecular impetus 

 impressed on them. Consequently the cell-proto- 

 plasm of the scion, while obtaining from the 

 stock all it needs within the limits of its own 

 variations of structure and activity, nevertheless 

 builds up and breaks down in ways or at rates 

 slightly different from those hitherto normal to 

 it, and perceptible variations result when the 

 sequences and correlations of these material and 

 mechanical changes have affected a sufficiently 

 large mass for the accumulation of visible effects. 

 The limits to grafting suggest not that an in- 

 appropriate stock does not offer to the protoplasm 

 of the scion the right materials, but that it pre- 

 sents them in proportions and in forms which are 

 unsuitable for the assimilable powers of the latter, 

 or, possibly, mingled with substances poisonous 

 in themselves or capable of becoming so in con- 

 junction with bodies in the scion. 



What has been said of the action of stock on 

 scion, will also be true, mutatis mutandis, of the 

 reciprocal action of scion on stock. Here again 

 we may have causes for disease, or predisposition 

 to disease. 



It occasionally happens, however, that the nu- 

 clear protoplasm of the stock or scion is affected 

 in grafting, and we infer from the difficulty of 

 modifying it in any other way in ordinary repro- 

 duction than b\- means of other nuclear protoplasm 



