288 



THE ANATOMY OF WOODY PLANTS 



is confined to the outside of the xylem. Endodermal layers, 

 both internal and external, are present, and the pith often contains 

 patches of brown sclerotic cells similar to those found in the cortex. 

 In spite of the fact that the bundles of the central cylinder of the 

 stem are collateral, those of the leaves are concentric in organiza- 

 tion. Applying the principles of comparative anatomy to the 

 situation, we find that in O. cinnamomea the pith and cortex 



are of common 

 origin and that the 

 bundles of the 

 stele, on the evi- 

 dence furnished by 

 the foliar strands, 

 were formerly con- 

 centric in structure. 

 The conclusion 

 which is reached 

 from the considera- 

 tion of the stem of 

 the species under 

 discussion, in the 

 light of universally 

 valid principles of 

 comparative anat- 

 omy, is that its 

 tubular stele was 



FIG. 205. Part of the stem of O. skidegatensis , still 

 more highly magnified to show the presence of both 

 internal and external phloem. 



formerly concentric and that the foliar gaps were once large 

 enough to permit of the joining of cortical and medullary tis- 

 sues with one another. The inferences drawn from the data 

 indicated above are, moreover, entirely justified by the considera- 

 tion of the anatomy of the Lower Cretaceous species described 

 in the preceding paragraph. In particularly vigorous specimens 

 of the stem in O. cinnamomea the ancestral condition, moreover, 

 frequently returns, for both open foliar gaps and internal phloem 

 are often seen in such axes. In 0. regalis among living repre- 

 sentatives of the Osmundaceae medullary brown sclerenchyma 

 is occasionally found in the pith, although an internal endo- 



