FORMATION OF WOOD 263 



of the stem. In Monocotyledons the structure 

 is different, but we are not concerned with them. 

 On the outside is a single layer of cells, with 

 thickened outer walls. The central cells are the 

 pith, the ring of groups of tissue is the pro- 

 cambium lying in the cortex, and the outer 

 layer of cells is the epidermis. The pith occupies 

 a considerable area in the cross section of a 

 young plant, but, in the case of timber trees -at 

 any rate, it either occupies so small a space in 

 the older parts as to be neghgible or its cells 

 die or decay away leaving the centre of the 

 stem hoUow. 



The procambium is a meristematic tissue, 

 that is to say, as we have explained above, a 

 tissue capable of forming new cells by division. 

 It has, moreover, the important property of 

 forming two totally different kinds of tissue. 

 On its inner side it forms wood or xylem, on its 

 outer side it forms bast or phloem. Between 

 these two tissues the meristematic tissue remains 

 and is now called fascicular cambium. 



The structure of the stem now in a still older 

 part, where the bundles of wood and bast are 

 just beginning to be formed, shows these essential 

 tissues. A central pith ; a ring of vascular 

 bundles, each consisting of inner wood, central 

 fascicular cambium and outer bast ; a tissue 

 called cortex, external to the ring of bundles, 



