CH. IV, 2] 



STRUCTURE OF STEMS 



125 



are outside, in reverse of the condition in the wood, as 

 shown in principle by our diagram (Fig. 81). The third 

 new feature consists in the SECONDARY MEDULLARY RAYS 

 (Fig. 82). They form in the ever-broadening fibro- vascular 

 bundles, which thereby are kept divided to nearly their 

 original width. It is hardly correct, however, to speak 



FIG. 81. Diagram of a cross section of a generalized stem, to illustrate 

 the interrelations of fibro-vascular bundles, pith, medullary rays, both 

 primary and secondary, cambium (black), cortex, and cork. Annual rings 

 in bark and wood of identical age are identically shaded. The extension of 

 the rings across the medullary rays is not shown, though it is usually plain 

 in the wood while obscure or absent in the bark. 



any longer of separate fibro-vascular bundles, since their 

 identity has long since been lost in that of the general woody 

 mass and the bark. 



The medullary rays are an important, and sometimes a 

 conspicuous feature of the wood. Beginning as plates of 

 tissue between the originally separate bundles, they are 

 later developed and multiplied in number as a persistent part 

 of the wood, in which they serve as avenues of communica- 



