210 



AMERICAN VINES, 



(a) Knitting Tissue. If the lower section of a cutting: 

 bearing one or two buds is placed under suitable conditions 

 of heat and moisture, there appears on its side, as in the 

 case of any other cuttings, small protuberances, termed callus 

 (Fig. 93), which are nothing but masses of healing tissue. 

 They are more numerous and more developed towards the 

 lower end of the section (Fig. 93) ; and on the splice of a 

 scion cut in the whip-tongue method the 

 callus appears first at the lower end of 

 the splice; on the side and upper ex- 

 tremity it only develops later on. The 

 object of these tissues is to cover the 

 wound made by this cut by a kind of 

 suberous (corky) layer, and to act as a 

 protection against outside influences (de- 

 cay, etc.) affecting the living part of 

 the graft (the cambium layer, liber, and 

 bark). The juxtaposition of a correspond- 

 ing section of the scion on the stock 

 modifies its natural destination. 



The wood has nothing whatever to do 

 with the production of healing tissue; its 

 action is nil, and it is never subjected to 

 cellular alteratives. But all parts of the 

 bark (liber), the cells uniting the medul- 

 lary rays to the fibro-vascular bundles, the 

 layers of medullary rays, the cells which 

 permeate the sieve tubes of the liber, the 

 Fig sieve tubes themselves, the cambium layer 



Callus or Knitting Tissue (the libriform fibres and exterior layer of 



developed on a scion. < , x M ,. 



cork excepted), contribute to its formation. 

 But the principal part in the formation of callus devolves 

 on the cambium layer. The mechanism of its formation is 

 as follows : 



The cells in immediate contact with the surface of the 

 section belonging to the parts named become more active, 

 subdivide, multiply, and become elongated in a direc- 

 tion approximately perpendicular to the surface of the cut. 

 The . liber cells and the soft liber become transformed into 

 soft, thin, and non-lignified cells ; they subdivide and mul- 

 tiply, and reunite with those produced with the activity of 

 the cambium layer, and constitute the layer or pad of heal- 

 ing tissue. Later on the exterior cells of the callus become 



