﻿198 RELATIONSHIPS. 



remains of the primary phloem. In addition to the elements named, scattered 

 idioblasts may occur in both the row and ray cells of various genera ; also spherical 

 aggregates of kliuorhombic (calcium oxalate) crystals. In Z. floridana these are 

 very numerous in the mesh rays and the small medullary rays of both xylem and 

 phloem. 



Finally, it remains to note that in Cycas, etc., just inside the origin of the main 

 portion of the xylem segments, interspersed in the pith, are groups of xylem con- 

 taining from a few to ten or more cells, in more or less regular rows. These lie in 

 nearly the same relative position as the centripetal xylem of primitive steles and, as 

 will be seen later, mark the innermost origin of the bundles passing out through 

 the mesh rays. 



In radial sections from adult trunks an absence of protoxylem is noted. The 

 tracheids of the secondary wood often have the normal bordered pits like those of 

 conifers on the radial walls {Cycas, Macrozamia, etc.), these sometimes grading into 

 scalariform structures in the same cell, a peculiarity noted in Cordaites Brandlingi 

 (111). Minute pittiugs may also be present in the tangential walls of these cells. 

 Rut in a medium-sized trunk of Zamia floridana the xylem wedges appear to 

 be entirely made up of spiral elements, and in Stangeria all the tracheids of this 

 secondary wood have scalariform pits. In Cycas revoluta the wood tracheids are 

 usually thickened by scalariform pittiugs, and but few have bordered pits, while 

 spirally thickened walls are absent. The most prominent objects in the phloem 

 are the greatly elongated fibrous cells, which distinctly add to the strength of a 

 stem so largely built up of parenchymatous tissues. Side by side with these are 

 the much elongated sieve tubes, on the oblique terminal and lateral walls of which 

 are numerous well-marked sieve plates of various shapes. The final prominent 

 features of the radial section are the large sheets of ray cells and the numerous 

 out-curving vascular bundles, the xylem and phloem of which mainly arise from 

 these same respective elements of the wood, traverse the mesh rays, and then 

 enter the cortex. Moreover, in describing the tranverse section, scattered xylem 

 groups were noted as occurring in the pith well within the main portion of the 

 xylem segments of the first vascular zone, and in the radial view these are seen to 

 consist in spiral to scalariform elements forming a loosely anastomosing xylem 

 system with each other and with the inner portions of the xylem segments, and at 

 the same time representing the innermost origin of the bundles passing out through 

 the mesh rays. Frequently in trunk forms duplicating the single original woody 

 cylinder these mesh bundles are in the same thin section traceable through several 

 vascular zones which have apparently grown around them. 



In tangential sections the fusion of the bundles into a more or less open and 

 regular network or trellis with vertically elongate meshes is clearly defined, as are 

 also the two kinds of medullary rays, that is, the few-seriate segment rays and the 

 broad mesh rays. In the lower angles of the mesh rays irregular strands of xylem 

 frequently arise, marking in part the origin of the leaf-trace or cortical bundles. Or 

 if the section be cut well out there is in each mesh a transversely cut leaf-trace 

 bundle with xylem directed upwards. Mucilage canals may also be present. In 

 Macrozamia there is in most of the meshes a single canal both above and below the 



