WOUNDS 



267 



liarities of a cow, nor does the stock assume the peculiarities of 

 the scion, although it is nourished by the metabolic products of 

 the latter. If the cambium cells of the stock naturally divide 

 more actively than those of the scion, the former will increase in 

 thickness more rapidly than the latter, and vice versa. The line 

 which marks externally the point of union between the fast- 

 growing and slow-growing portions of the stem has been called 

 by Goppert the " External line of demarcation," and is often 

 recognizable by distinctions in the cortex and bark. Internally 

 there is of course also 

 a corresponding line of 

 demarcation, along which 

 the wood of the stock 

 and scion unite, and 

 which may often be re- 

 cognized by a difference 

 in the colour of the wood 

 (Fig. 152). 



Many cases are known 

 where it must be ad- 

 mitted that the scion 

 exerts an influence on 

 the stock. It has been 

 observed, for instance, 

 that when a scion with 

 variegated leaves has 

 been used, variegated 

 leaves have sometimes 

 been produced on the 



shoots that have afterwards formed on the green-leafed stock. 

 Such a case forces us to the conclusion that the plastic materials 

 produced in the variegated leaves of the scion possess peculiar 

 properties which act upon the cambium cells of the stock in 

 such a way as to induce variegation in the leaves of the new 

 shoots. It is not my intention now to discuss the still more 

 potent influences which the scion has been known to exert in 

 certain cases on the stock, merely remarking that hybrid forms 

 have been obtained by grafting different varieties of potatoes on 

 each other. 



FIG. 152. Transverse section through the region 

 where Sorbns Aria has been grafted on 

 S. Aucuparia. The boundary line, a a, be- 

 tween the slow-growing S. Aria and the fast- 

 growing S. Aiicuparia is known as the in- 

 ternal line of demarcation. One fourth natural 

 size. 



