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THE ANATOMY OF WOODY PLANTS 



nests of cells which are duplicated by similar structures in the 

 cortex. The inner surface of the cylinder of xylem shows clusters 

 of primary wood which sharply contrast with the secondary 

 region by the irregularity of the arrangement of the elements and 

 the absence of rays. Fig. 211 illustrates a portion of the cylinder 

 more highly magnified, and with the greater enlargement the 

 distinct and mesarch character of the primary region becomes 

 apparent. The secondary wood is characterized both by its 

 regularly radial arrangement and by the presence of rays which 



FIG. 210. Transverse section of the stem of Lyginodendron oldhamium (after 

 Scott). 



are often two or three cells in width. The tracheids of secondary 

 origin are marked by crowded pits which alternate in position 

 and are often angular from mutual contact. These pores are 

 confined strictly to the radial walls of the elements, no tangential 

 pitting occurring in the secondary tracheids of any Paleozoic 

 gymnosperms with which we are at the present time acquainted. 

 The foliar traces in Lyginodendron are at first single, but bifurcate 

 shortly after leaving the primary region of the stem, and lose 

 their secondary xylem, becoming concentric instead of collateral 

 in their organization. It is evident on the basis of the general 

 canons earlier elucidated that the bundles of the stem in the 

 case of Lyginodendron must formerly have been concentric in 

 their structure, since this condition persists in the traces of the 

 leaves. Further, the presence of secondary wood in the stem 



