4 02 SCIENCE PROGRESS. 



A. The principal medullary rays remain parenchymatous 

 throughout the. whole thickness of the secondary wood. 

 B. The principal rays disappear, as such, towards the 

 exterior, owing to the formation of interfascicular wood. 1 



The retention of Williamson's genus Calamopitus 2 is 

 thought advisable in view of the peculiar structure of its 

 medullary rays, and other distinctive features. The type 

 of structure characteristic of Calamodendron, as described 

 by Renault and others from France and Saxony, has not so 

 far been discovered in England. 



Turning to longitudinal sections, we find many important 

 facts as regards the course of the xylem strands and their 

 behaviour at the nodes. The course of the bundles " is 

 essentially that of Equisetum. If we trace any bundle from 

 below upwards, we find that at the node it bends out in a 

 horizontal direction, forming the foliar bundle, which is 

 therefore cut transversely in a tangential direction." 3 At 

 each node the numerous anastomosing bundles form a pro- 

 minent mass of nodal xylem, which is seen to arch inwards 

 towards the diaphragms ; it consists largely of short scalari- 

 form tracheae, and internally of the innermost elements of 

 the primary xylem. The diaphragms uniting the nodal 

 wood across the fistular pith consist of parenchymatous 

 cells, in which there occur distinct traces of meristematic 

 divisions parallel to the upper and lower surface, suggesting 

 the formation of corky tissue. It would appear that the 

 elements of the xylem are tracheids, and not vessels, each 

 being developed from a single mother-cell. In looking at a 

 tangential section through the secondary wood in the neigh- 

 bourhood of a node, one sees alternating strands of xylem 

 tracheids and comparatively broad primary medullary rays ; 

 at the nodes the former bifurcate and fuse with the branches 

 of strands below the node. The upper ends of the rays 

 immediately below a node show fairly clearly a well-defined 

 group of smaller isodiametric cells, some of which may have 

 thickened walls. These infranodal patches of the ray tissues 



1 Williamson and Scott, p. 878. 

 - Williamson (1). :; Williamson and Scott, p. S75. 



