THE EQUISETALES (INCLUDING SPHENOPHYLLALES) 269 



tangential aspects. True wood parenchyma did not occur in the 

 secondary wood of Catamites. Although the primary xylem of 

 calamitean forms has departed far from the primitive condition, 

 the secondary ligneous organization of the group is characterized 

 by features which are only less primitive than those presented by 

 the lepidodendrids. 



The organization of the 

 leaf in Calamites is not well 

 known as regards those 

 features which are of inter- 

 est from the comparative 

 anatomical standpoint, but 

 this situation is fortunately 

 relieved by certain data 

 observed in the case of the 

 foliar organs of the living 

 genus Equisetum, which 

 will be described in a sub- 

 sequent paragraph. The 



root of calamitean forms 



riG. 192. iransverse section of stem of 

 was not recognized at lirst Equisetum variegatum var. Jesupi. 



as belonging to the forms 



included under this appellation and was called Astromydon. Its 

 organization can best be discussed in connection with that of the 

 root of the living genus Equisetum. 



In Fig. 192 is portrayed the structure of a transverse section of 

 the stem of the genus Equisetum. It is evident that the center of 

 the figure is occupied by a large air space shared by the calamitean 

 forms and indicative both for the Equisetaceae and their fossil 

 forebears, the Calamitaceae, of a primitively amphibious habitat. 

 This central cavity is often called the medullary fistula and in the 

 case of the ancient representatives of the Equisetales was often 

 molded in stone as pith casts resulting from mud entering the 

 central spaces of the fallen trunks rotting in the shallow waters of 

 Paleozoic lakes. Surrounding the large medullary space are the 

 fibro vascular bundles, which are of small size and somewhat remote 

 from one another. The strands are marked by cavities in their 



