[Chap. XXXV VEGETATIVE MULTIPLICATION 417 



formation in cuttings of certain varieties in which the initiation of roots 

 is very slow.^ 



The relative amount of shoot and root growth from a cutting is affected 

 by the presence of carbohydrates and proteins within it. Both grow well 

 if there is a good initial supply of each of these foods. But if carbo- 

 hydrates are abundant and proteins are scarce, root growth is much bet- 

 ter than shoot growth. 



The physiological condition of cuttings greatly influences their be- 

 havior. If the plant is dormant, the cuttings do not live unless they are 

 kept in cool moist conditions until adventitious roots have begun to 

 develop. If the plant is just beginning to bear flowers, the cutting may 

 grow but little and then bear flowers. 



Grafting. Grafting is an artificial method of promoting vegetative 

 propagation when cuttings do not root readily, and when the plants do 

 not reproduce true from seeds. Most varieties of trees cultivated for their 

 fruits are perpetuated by some method of grafting. 



Grafting consists in attaching a cutting, the scion, to the root, stem 

 base, or branches of another plant, the stock. The scion ma\^ be a twig 

 bearing several buds, or it may be a single bud attached to a small piece 

 of bark. If two plants are growing close together the bark may be re- 

 moved from convenient points on branches or main stems, which mav 

 then be bound together at these points. This is called approach grafting. 

 When the union between the stems is complete, the parts of either that 

 are not desired may be cut away. 



Valuable trees that have been girdled by animals or by extremes of 

 temperature may be saved by bridge grafting, in which opposite ends of 

 each of several twigs are inserted as grafts beneath the bark on the upper 

 and lower edges of the girdle. 



The parts of plants used in grafting are illustrated by diag^ams in 

 Fig. 193. The cambiums of the stock and scion must be brought into 

 close contact. The grafted sections are then bound together with a string, 

 and the junction is covered with waxed tape to prevent drying and the 

 entrance of destructive organisms. 



The cambium and other meristematic cells at the cut surfaces grow 



^ Three of these compounds most \'aluable in promoting plant propagation on a com- 

 mercial scale are indole butyric acid, naphthalene acetic acid, and indole acetic acid. 

 Others important scientifically are indole propionic acid, ethylene, propylene, acetylene, and 

 carbon monoxide. Directions for applying these compounds to cuttings of different vari- 

 eties of plants may be found in publications from the Boyce Thompson Institute of Plant 

 Research, and also in circulars from chemical supply companies that market them under 

 such trade names as "hormidin," "auxillin," and "rootone." 



