152 



THE ANATOMY OF WOODY PLANTS 



the angles of protoxylem become confluent by continued centripetal 

 development has interspersed with its elements a varying amount 

 of parenchyma. Vessels are present in the primary five-angled 

 region of the wood, but they do not so conspicuously differ from 

 the tracheids in transverse section as is the case with the similar 

 elements of the secondary wood. 



Two features are particularly worthy of note in the present 



connection. First of 

 all, the layer of peri- 

 derm does not occupy 

 so deep a position in 

 the root as in the larch 

 and fir shown above. 

 As a consequence the 

 central cylinder does 

 not generally become 

 stripped of its cortical 

 envelope as has been 

 found in the conifers. 

 Another interesting 

 feature is presented by 

 certain modifications 

 in the rays in relation 

 to the star-shaped 

 mass of primary wood. 



This situation is more easily understood by reference to the 

 accompanying photographs. The first illustration (Fig. 108) 

 represents a transverse section of a mature root of Alnus 

 japonica. The secondary wood is characterized by certain 

 modifications in the structure of the radii corresponding to 

 the angles of the primary wood. These consist of clusters, more 

 or less pronounced, of rays of greater width than the uniseriate 

 condition found in the wood at large, among which the vessels, 

 characteristic of the organization of the remainder of the xylem, 

 are absent. In other words, aggregate rays are plainly present in 

 the secondary wood and are quite clearly related to the groups 

 of protoxylem marking the angles of the primary xylem. Since 



FIG. 108. Transverse section of root of Alnus 

 japonica. 



