X] 



ARTHRODENDRON. 



327 



form of the principal medullary rays is seen in fig 83, C, and 

 the reticulate pitting on the radial wall of a tracheid is shown 



Fig, 83. Calamites (Arthrodendron) . 



A. Tangential section (diagrammatic) showing the course of the vas- 



cular strands, also leaf-traces and infranodal canals. 



B. Radial face of a tracheid. 



C. Prosenchymatous elements of a principal medullary ray. 



D. Transverse section of the wood. (After Williamson.) No. 36 in 



the Williamson Collection. 



in fig. 83, B. Fig. 83, A illustrates the large infranodal canals 

 as seen in a tangential section of a stem. The same section 

 shows also the course of the vascular bundles characteristic of 

 Calamites as of Equisetum, and the position of outgoing leaf- 

 traces is represented by unshaded areas in the black vascular 

 strands. 



The subgenus Arthrodendron is very rarely met with, and 

 our information as to this type is far from complete'. 



The third subgenus Calamodendron has not been discovered 

 in English rocks, and our knowledge of this type is derived from 



Williamson and Scott (94), p. 879. 



