306 CALAMITES. [CH. 



formed in precisely the same manner as the corresponding 

 spaces in an Equisetum stem, and they are spoken of in both 

 genera as the carinal canals. Each canal owes its origin to the 

 disorganization and tearing apart of the protoxylem elements 

 and the surrounding cells. This may be occasionally seen in 

 examples of very young Calamites ; the canals of a young twig 

 often contain apparently isolated rings which are coils of 

 elongated spiral threads. Fig. ^2, B represents the canal of a 

 twig, cut in an oblique direction, in which the remains of spiral 

 tracheids are distinctly seen. In the stem of fig. 71 the 

 development has not advanced far enough to enable us to 

 clearly define the exact limits of each xylem strand. The 

 smaller elements bordering the canals constitute the primary 

 xylem, they are fairly distinct on the outer margin of some 

 of the canals seen in the section. Between the small patches 

 of primary xylem the outward extensions of the parenchyma 

 of the pith constitute the primary medullary rays, mr. The 

 distinct line encircling the canals and primary xylem has been 

 described by Hick as marking the position of the endodermis, 

 but it may possibly owe its existence to the tearing of the tissues 

 along the line where cambial activity is just beginning. This 

 layer of delicate dividing cells would constitute a natural 

 line of weakness. External to this line we have a zone of 

 tissue a, d, containing here and there larger cells with black 

 contents, which are no doubt secretory sacs. It is impossible to 

 distinguish with certainty any definite phloem groups, but in 

 other specimens these have been recognised immediately ex- 

 ternal to each primary xylem group ; the bundles were typically 

 collateral in structure. Towards the periphery of the twig the 

 preservation is much less perfect; the outer portion of the 

 inner cortex, d, consists of rather smaller and thicker-walled 

 cells, but this is succeeded by an ill-defined zone containing a 

 few scattered cells, 6, which have been more perfectly preserved. 

 The twig is too young to show any secondary tissue in the cortex ; 

 but the tangential walls in some of the cortical cells afford evidence 

 of meristematic activity, which probably represents the beginning 

 of cork-formation. The limiting line, e, possibly represents the 

 cuticularised outer walls of an epidermal layer. The irregularly 



