346 CALAMITES. [CH. 



Equisetum roots, the cells of these layers are placed on the 

 same radii, as seen in the neighbourhood of x^ in the figure. 

 This correspondence of the radial walls of the endodermal and 

 pericyclic cells points to the development of both layers from 

 one mother- layer, and suggests the 'double endodermis' or 

 phloeoterma of Equisetum (p. 254). The cells in the outer of 

 these two layers have slight thickenings on the radial walls 

 recalling the usual character of endodermal cells. The phloeo- 

 terma is succeeded by a few layers of parenchyma, constituting 

 the inner cortex, and beyond this we have the large lacunae 

 separated from one another by slender trabeculae of cells. The 

 outer cortex is limited by a well-defined layer of thick-walled 

 cells, which may be spoken of as the epidermoidal^ layer. 

 Roots possessing this superficial layer of thicker cells have no 

 doubt lost the original surface-layer which produced the 

 absorptive root-hairs. 



The xylem elements have the form of spiral, reticulate 

 and scalariform tracheids. 



In roots or rootlets smaller than that shown in fig. 91 B, the 

 primary xylem may extend to the centre of the stele, and form 

 a continuous axial strand ; in such examples the structure may 

 be diarch, triarch or tetrach. The origin of the cambium 

 agrees with that in recent roots, the cells immediately external 

 to the protoxylem tracheids become meristematic, as also 

 those internal to the phloem. Another root-character is seen 

 in the endogenous origin of lateral members. Good examples of 

 branching roots are figured by Williamson^ and by Williamson 

 and Scott I 



Older roots* are usually found in a decorticated condition. 

 A transverse section of root in which secondary thickening has 

 been active for some time presents on a superficial view a close 

 resemblance to a stem of Catamites, but a careful comparison 

 at once reveals important points of difference. The specimen 



^ Williamson and Scott (95), p. 694. 

 - Williamson (832), pi^ ^xix. fig. 7. 

 •^ Williamson and Scott (95), Pis. xv. — xvii. 



"* For figures vide Williamson, loc. cit. , Williamson and Scott, and Eenault 

 (85), (93). 



