270 Propagation by Layers and Grafts. 



portion of which is supplied by the graft to the stock for the ex- 

 tension of its own tissues, just as if the stem really belonged i [♦ 

 To effect this object, it is generally necessary to choose as 

 the stock, a plant either of the same species as the graft, or 

 one very closely allied to it ; and the less the relationship, ^ 

 more care and precaution must be taken to secure a union, by 

 bringing the newest layers of bark and wood into contact. It 

 is customary to select for the stock some less valuable form of 

 the same species. Thus the cultivated varieties of Pears and 

 Apples are often grafted upon the Wild Pear and Crab. Or 

 a species nearly allied, will sometimes answer nearly as well 

 and from being readily procured, is commonly employed.- 

 Thus, Peaches and Apricots are grafted on the common Pl ura , 

 The operation does not always succeed between two species 

 of different genera ; and it fails entirely, if an attempt is made 

 to unite individuals of different families. Thus, for example, 

 Pears answer well upon Pears, nearly as well upon Quinces, 

 less freely upon Apples or Thorns, and not at all upon Pk, ms 

 or Cherries, which are of a different family. The Lilac will 

 take upon the Ash, notwithstanding their great apparent differ- 

 ence, because they are of the same natural family ; but the 

 Olive, which also belongs to the same family, cannot be profit- 

 ably grafted upon the Ash, since the vegetation of these is too 

 different to allow them to live long together. 



The cause which thus restrains the gardener in the choice 

 of his stock, is not merely the difference in the properties of 

 the fluids of the two kinds, but also the difference in the gene- 

 ral character of their growth. It is essential that the stock and 

 graft should be naturally in sap at the same time ; and this is 

 more likely to be the case in nearly allied species than in 

 others. However, in very succulent plants, such as the Cacti, 

 of which the fleshy stems are always full of fluid, grafts of very 

 different species succeed very well together ; and this excep- 

 tion helps to prove the rule. It is necessary, also, that the rate 

 of growth of the two should be nearly the same ; for, if the 

 graft be of more rapid growth than the stock, and more be 

 sent down to the latter than it can convert into tissue, a swel- 

 ling will be formed above the line of union, like that which 



O 



takes place when a cord is bound round a stem ; and this will 



