92 STRUCTURAL BOTANY 



procambial strand, is called the fascicular cambium ; 

 that between the bundles, formed by division of the 

 cells of a primary medullary, ray is the interfascicular 

 cambium. 



The interfascicular wood, i.e. that formed by the 

 parts of the cambium between the bundles, consists at 

 first chiefly of fibrous cells and parenchyma, but farther 

 to the outside vessels are formed here also. The limits 

 between the original bundles can, however, always be 

 traced, for the cambium produces a radial band of par- 

 euchymatous tissue two or more cells thick, which extends 

 right through from pith to pericycle ; this is called a 

 principal medullary ray (see Fig. 35). It corresponds 

 in position with a primary ray, and forms, as it were, 

 the continuation of it, but is much narrower, for the 

 cambium only forms the ray parenchyma just opposite 

 the middle of the primary ray. The cells of the ray 

 are short, with square ends ; their walls are thick and 

 pitted (see Fig. 36, mr); they form starch in summer, 

 and are easily distinguished by their shape from any 

 other cells of the wood. The part of the ray which 

 is formed on the outside of the cambium broadens out, 

 its cells becoming both larger and more numerous. 

 Between the principal medullary rays there are shorter 

 secondary ones (Fig. 35, s.m.r), which are started by 

 the cambium after growth in thickness has gone on 

 for some time, and which therefore do not extend 

 either inwards to the pith or outwards to the pericycle. 

 The secondary as well as the principal rays broaden 

 out in the phloem. As the wood constantly increases 

 in thickness, it is evident that all the tissue outside it 



