98 FOUNDATIONS OF BOTANY 



growth must take place in the manner shown in Fig. 69. 

 All the cambium, both that of the original wedges of wood, 

 fc, and that, ic, formed later between these wedges, con- 

 tinues to grow from its inner and from its outer face, and 

 thus causes a permanent increase in the diameter of the stem 

 and a thickening of the bark, which, however, usually at 

 an early period begins to peel off from the outside and 

 thus soon attains a pretty constant thickness. 1 It will be 

 noticed, in the study of dicotyledonous stems more than a 

 year old, that there are no longer any separate fibre-vascular 

 bundles. The process just described has covered the origi- 

 nal ring of bundles with layer after layer of later formed 

 wood-cells, and the wood at length is arranged in a hollow 

 cylinder. 



It is the lack of any such ring of cambium as is found 

 in dicotyledonous plants, or even of permanent cambium 

 in the separate bundles, that makes it impossible for the 

 trunks of most palm trees (Fig. 54) to grow indefinitely 

 in thickness, like that of an oak or an elm. 2 



109, Grafting. When the cambium layer of any vigor- 

 ously growing stem is brought in contact with this layer 

 in another stem of the same kind or a closely similar kind 

 of plant, the two may grow together to form a single stem 

 or branch. This process is called grafting, and is much 

 resorted to in order to secure apples, pears, etc., of any 

 desired kind. A twig from a tree of the chosen variety is 

 grafted on to any kind of tree of the same species (or some- 

 times a related species), and the resulting stems will bear 

 the wished-for kind of fruit. Sometimes grafting comes 



1 See Vines' Students' Text-Book of Botany, London, 1894, pp. 211, 212. 



2 See, however, Strasburger, Noll, Schenk and Schimper's Text-Book, 

 pp. 138, 139. 



