[94 THE PHYSIOLOGY OF PLANTS 



.^ 37. To what extent do scion and stock mutually influence 

 one another? 



As long as it was supposed that cambium was a sort of sap 

 which circulated between the bark and the wood, it was firmly 

 believed that the characters of scion and stock became gradually 

 intermingled. Now, however, we know that the cambium is a 

 tissue, the young cells of which have inherited from their first 

 formation the tendencies of their mother-cells, and therefore 

 continue to function in the same way, forming the same sort 

 of cell-wall and cell-contents as their predecessors did. The 

 characters of the scion as well as those of the stock will develop 

 themselves separately in their several tissues. 



But it cannot be denied that in those places where the 

 callus cells of scion have united with those of the stock, the 

 cells adjoining each other and belonging to two different plants 

 will influence each other to a certain extent. Each cell pre- 

 pares its substances in its own peculiar way, and those of its 

 contents which diffuse out into the neighbouring cells, or, in 

 case of the sieve tubes, pass over in a mass, will cause a dif- 

 ference of nutrition, and such a difference of nutrition will be 

 connected with differences of character. It is, therefore, not 

 strange that we should be able to demonstrate the influence of 

 scion and stock one upon the other. But a fusion of char- 

 acters never takes place, because the main functions of each 

 cell are too well fixed by heredity to be able to be changed by 

 mere differences of nutrition. 



But apart from these considerations, the grafting will entail 

 a certain mechanical effect, because in the new individual, 

 consisting of the two parts, a transverse layer of short wood 

 parenchyma has arisen. 



This layer causes a certain amount of difficulty to all con- 

 duction. The wood vessels of the wild stock, as far at least 

 as they were formed before the grafting was effected, are not 

 directly continued into the scion, because this layer of callus 

 lies between them. And even when the fusion of tissues has 

 taken place, and the continuous zone of cambium has formed 

 uninterrupted vessels which pass through the juncture of the 



