xxxiv] COENOXYLON 293 



together with the Middle Devonian species Cordaites Newberryi 1 

 in Zalessky's genus. Though these two American species are 

 comparable in the discontinuous arrangement of the tracheal pits 

 with the Russian type the latter is characterised by the presence 

 of primary xylem-strands, a feature that has not been recognised 

 in the American stems: it would seem, then, undesirable to 

 adopt the designation Callixylon in preference to Dadoxylon 

 unless there is evidence as to similar characters in the primary 

 region of the xylem. 



Coenoxylon. Zalessky. 



Coenoxylon Scotti Zalessky. Prof. Zalessky 2 gave the name 

 Coenoxylon to a small and incomplete piece of stem of doubtful 

 provenance but possibly from the Ural Permian beds. The pith, 

 2 cm. broad, consists of parenchyma associated in the central 

 region with numerous large sclerous cells. In one section a 

 sinuous band of meristematic tissue was observed near the 

 periphery of the pith : the appearance of this tissue in a photo- 

 graph given to me by Prof. Zalessky suggests comparison with 

 occasional strips of similar dividing cells in the pith of Lyginopteris. 

 The secondary wood is composed of tracheids with 1 2 rows of 

 flattened or hexagonal pits on the radial walls and narrow uni- 

 seriate medullary rays reaching a depth of 15 cells and with 2 7 

 oval pits in the field. As in the wood of Mesopitys Tchihatcheffi 3 

 there are distinct and apparently complete rings of growth. 



It is on the ground of the arrangement and structure of the 

 primary xylem that Zalessky instituted a new generic name. 

 The primary xylem forms teeth of variable size which project 

 into the pith from the edge of the secondary xylem: the promi- 

 nent portions of the main mass of primary xylem give off branches, 

 differing considerably in size and shape, some of which become 

 separated by a comparatively broad band of parenchyma from 

 the parent xylem-tissue. These bundles anastomose in their 

 course through the pith and in doing so incorporate between them 

 patches of parenchyma. The bundles of primary xylem are 

 endarch. From the centrifugal strands at the periphery of the 



1 Penhallow (00) pp. 64, PI. xn. 2 Zalessky (II 2 ) p. 13, figs. 14. 



3 See page 295. 



