82 NUT GROWING 



apple trees in this regard and present a phase of 

 grafting by hybridization perhaps. In an older top- 

 worked tree, however, with an established root sys- 

 tem, the hormones elaborated in the top presumably 

 find greater difficulty in changing the character of 

 the root. When grafting different species of nut 

 trees it will be well to bear in mind that varieties or 

 species for union purposes should have wood of ap- 

 proximately similar texture. When the stock and 

 scion unite the union is mechanical in its nature 

 rather than physiological. The different life types 

 or character of the scion and stock do not fuse as a 

 rule. If the type of cell in the stock is very different 

 from the type of cell in the scion the union is not 

 satisfactory either for transformation of food ma- 

 terial or for stability of union. 



In northern temperate climates the chosen time 

 for scion grafting has usually been in the early 

 spring while bud grafting has been done in the latter 

 part of summer. In the springtime the cambium 

 layer of the bark is very actively engaged in con- 

 ducting new growth and making repair of wounds. 

 During mid-summer and early fall division of cam- 

 bium cells goes on tardily, but in the buds themselves 

 there is at this time a larger embryonic growth than 

 there is in the spring. The separate seasons for 

 scion grafting and for bud grafting date back to a 

 period so remote that in all probability there was no 

 question of plant physiology in the minds of the 

 older horticulturists. Doubtless a general recogni- 

 tion of the fact of spring activity in plants served 



