RELATIONS OF CALAMODENDRON TO CALAMITES. 365 



Iq fig. 4 [Calamites) we find that the cells of the 

 broad medullary ends of the two primary medullary rays 

 g,g, are larger in size and less regular in their arrange- 

 ment than those of the narrower, more peripheral portion 

 g^, of each ray, where the cells are smaller in size and dis- 

 posed in regular radial rows, parallel to those of the vessels 

 of the vascular wedge, c. Turning to the tangential side 

 of the block, we see that the vertical extensions of the 

 same rays, g" , g", are still composed of parenchyma, the 

 component cells of which tend to assume an arrangement 

 in vertical lines. 



Between these two rays we have part of a vascular 

 wedge, c, narrower at its medullaiy end than at its 

 opposite one. It is composed, as is most usual, of barred 

 vessels or tracheids, not always easily distinguishable in 

 transverse sections from the cells of the more peripheral 

 extremities of the primary medullary rays. In the tan- 

 gential section, we see the secondary medullary rays, /, of 

 the wedge, each being composed of variable numbers of 

 cells arranged in vertical rows. 



Turning to a similar diagram of a cubic block from my 

 Calamopitus, fig. 5, we find the general arrangements to 

 be identical with those of fig. 4. The differences between 

 them are chiefly twofold. In this plant, the transverse 

 section shows the cells g, g, of the two primary medullary 

 rays to be more uniform in size and more regular in their 

 linear, radial arrangement than is usual amongst the 

 Calamites. This exceptional condition exists close to the 

 medullary axis as well as more peripherally, as will be 

 seen on contrasting fig. 4, g, g, with fig. 5, g, g. But the 

 most striking feature in this second type is seen in tan- 

 gential sections of these rays, as at fig. 5, g', g'. Instead 

 of being composed of an aggregation of parenchymatous 

 cells, these rays consist of a very marked pronenchijmatous 



