FIB RO VASCULAR TISSUES: RAYS 



8 1 



ous diagram given previously, are represented in black on the 

 periphery of the stem. The central pith is likewise encircled in 

 turn by wood, phloem, and cortex. Here, too, there are excur- 

 sions of the pith at six points, extending into the second annual 

 increment of the wood; these subtend radially six corresponding 

 leaves. In the diagram under discussion there are, however, two 

 marked features of contrast to those presented in the foregoing 

 scheme of a coniferous axis. First of all, the wood is character- 

 ized by the presence of vessels, and 

 secondly by a broad radial stripe 

 which extends from each leaf gap 

 outward. This radial stripe con- 

 trasts with the rest of the woody 

 cylinder by the absence of vessels 

 and the clustering of rays. The 

 radial parenchymatous stripes which 

 lie in the region of the six radial 

 bands corresponding to six leaves are 

 not only more prominent than the 

 linear and somewhat sparse rays in 

 the rest of the wood but are of 



greater width. The aggregations of rays related to the leaves 

 shown in the diagram are, in fact, clustered rays of the type exem- 

 plified by C. torulosa (Fig. 59) and are, as a consequence, aggregate 

 rays. Since they are in this instance clearly related to leaves, 

 they may at the same time be appropriately designated foliar 

 aggregate rays. 



With the exposition of the essential features of organization 

 present in coniferous and dicotyledonous stems, respectively, we 

 are in a position to proceed further with the highly important 

 discussion of the evolution of the radial parenchymatous structures 

 of dicotyledonous woods. Simplicity will be served and ambiguity 

 avoided if in further elaboration we hold to the conception of the 

 rays as they present themselves in a small twig of a few years' 

 growth. Fig. 63 illustrates synoptically the main types of rays in 

 the dicotyledons as seen in small branches. The x>nly essential 

 departure from the conditions in nature is the delineation of the 



FIG. 62. Diagrammatic trans- 

 verse section of the stem in Casua- 

 rina. Explanation in the text. 



