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, 
fe, and that, zc, 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. It will be 
noticed, in the study of dicotyledonous stems more than a 
year old, that there are no longer any separate fibro-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.” 
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. 158, 139. 
