VEGETATIVE MULTIPLICATION 



415 



[Chap. XXXV 



pericycle or from cells in the phloem or cambium of the stem (Fig. 191 ) . 

 They do not develop readily in stem cuttings of some plants, and not at 

 all in others. 



'7l-\ f-'i 



- 't*. 



Fig. 191. Cross section of a root of Cissm in which lateral root primordia have 

 developed from cells in the pericycle at points directly external to the vascular 

 bundles of the main root. Photomicrograph from N. E. Pfeiffer, Boyce Thompson 

 Institute. 



Stem cuttings may be placed directly in soil, moist sand, peat, and 

 other media. They may also be grafted on the stem or roots of another 

 plant. 



When a stem cutting is placed in a suitable "rooting medium," roots 

 grow from the morphologically basal end, and new shoots develop from 

 the apical buds. The development of these apical shoots prevents the 

 growth of shoots from buds further down the stem. Likewise the growth 

 of basal roots prevents the growth of roots farther up the stem, even 

 though preformed root primordia are present. This apical dominance is 

 so pronounced in both roots and stems that it is seldom reversed, except 

 by drastic changes in the physiological condition of the cuttings. 



The origin and growth of the roots depend upon sugar and hormones 

 and perhaps vitamins from the leaves (Chapter XX). Consequently 

 when cuttings are made of herbaceous plants ("slips"), or of the young 

 stem segments of woody plants ( soft-wood cuttings ) , the origin of roots 

 and their rate of growth are influenced by the leaves left on the cuttings 



