GRAFTS. 265 



mation of callus at the cut surfaces, and the 

 intimate union of the ingrowing cells from each 

 callus. 



The development of the callus follows the 

 course described for wounds, cuttings, etc., and the 

 union is exactly comparable to the union of the 

 two lips of a healing callus over a wound (see 

 p. 197). 



Grafting was known and practised far back in the 

 ages. Virgil was well acquainted with the process, 

 and Theophrastus compared it with propagation by 

 cuttings. 



The scion differs from a cutting, however, in 

 having no roots of its own : it is parasitic upon, or 

 rather is in symbiosis with the stock, the root and 

 tissues of which intervene between it and the soil. 

 Consequently the selective absorption, size and 

 number of vessels, and innumerable other physio- 

 logical and anatomical peculiarities of the stock 

 determine what and how much shall go up into 

 the scion, while the latter supplies the former with 

 organic materials and rules what and how much 

 food, enzymes, and other secretions, etc., it shall 

 receive to build up its substance. Surely, then, if 

 such factors as the nature of the soil, the water and 

 mineral supplies, the illumination, and the various 

 climatic factors of altitude can cause variations on 

 a plant direct, these and other factors are still 

 more likely to be effective on stock and scion, and 

 each must affect the other. 



Nevertheless opinions have differed much as to 

 whether any important effect is to be seen, and on 



