350 The Living Plant 



stock, must be closely related, else no union of tissues takes place. 

 We find the same necessity in hybridization, or crossing of dif- 

 ferent varieties of species by pollination; and indeed the possibili- 

 ties of grafting and of hybridization have much the same limits, 

 being comparatively easy between varieties of one species, much 

 less so between species of the same genus, extremely rare between 

 different genera, and unknown outside of the same family. Prob- 

 ably the reason is a chemical one — the more distantly related the 

 forms the more likely is their protoplasm to contain chemicals 

 which react on one another in a way to produce disturbing if not 

 injurious or fatal compounds, thus preventing a normal or orderly 

 continuance of growth. But when the protoplasm of scion and 

 stock is actually congenial, so to speak, then the two grow to- 

 gether precisely as a wound on one plant would heal up, and the 

 tissues unite and thereafter grow as one single mass. It is neces- 

 sary that a considerable area of living tissue be brought into con- 

 tact, which is comparatively easy in these plants possessing a 

 cambium cylinder (i. e. a continuous growth system soon to be 

 described), but it is practically impossible in others. This fact 

 explains why no grafting is possible among plants belonging to the 

 groups of the Corn, Lilies, Palms, wherein no cambium exists. 



Although, in general, the scion and stock retain each its own 

 characters unaffected by the other, a partial exception occurs in 

 some minor features, such as earliness of blossoming, resistance to 

 frost, and even some slight alterations in flavor of fruit or its 

 color. In all these cases, I believe, such characters can be traced 

 to the influence of the sap, which of course moves from stock to 

 scion, or of the food substance, which moves from scion to stock. 

 The living protoplasm, however, does not thus move from one to 

 another, but remains within the original cells, or those which 

 grow from them; wherefore the characters which depend on the 

 protoplasm, including substantially all of those which give the 

 distinctive characteristics to plants, are never transferred from 

 stock to scion, or vice versa. 



