On the Fossil Plants of the Coal-measures. 229 



One specimen of the vascular axis is, as already mentioned, so 

 large as to demonstrate that the plant became arborescent. 



Though JJidi/o.i'ulon was not a dichotomizing plant, like Lepido- 

 dendron, it gave off lateral bundles of vessels. JSome of these are 

 simple bundles, consisting of numerous vessels intermingled with 

 some cellular tissue. Others have this central bundle invested by 

 a thin ring of radiating laminae with intervening medullary rays ; 

 this exogenous ring sometimes becomes developed into a relatively 

 large and distinct woody zone, Hke that of the parent axis. The 

 vessels of these lateral growths appear to be wholly derived from 

 the radiating woody zone. 



A second form of lateral appendage appears to spring from the 

 medullary rays, and consists of a cyhudiical mass of reticulated 

 cells, which are chiefly prosenchymatous, but of an elongated type. 

 It is suggested that this structure may have been prolonged into an 

 adventitious root. 



The structure of the central or medullary vascular axis of the 

 former of these two kinds of lateral appendages seems to indicate 

 that the history of the development of the medullary vascular cylin- 

 der in these plants corresponds with what the author described 

 in his preceding memoir (Part III.) as taking place in the similar 

 parts of the Lejndodendra, viz. that some of the cells of the central 

 part of the axis underwent rapid fission, and thus developed a di- 

 stinct cellular medulla, which toreed the medullary vessels outwards 

 where at first they constituted a liug, but which ring soon broke 

 up into the detached vascular masses already referred to as adheiing 

 to the inner surface of the exogenous zones. 



The enlargement of the exogenous woody cylinder by the peri- 

 pheral intercalaticm of new radiating vascular lamin?o, and the 

 repeated subdivision of these laminje by a corresponding intercala- 

 tion of new medullary rays, demonstrates the close resemblance 

 between the growth of the ligneous zone in these plants and that 

 of ordinary exogenous stems. A fine series of specimens collected 

 by the Hev. H. Higgins, of Rainhdl, near Liverpool, and which 

 exhibit various modifications of the type figured by the late Mr. 

 Gourlie under the name of Lijr/inodcndron Landshun/hii, are shown 

 to be merely casts of the exterior surface of the bark of some species 

 of Didi/oxi/lon. They may actually belong to D. Oldhamium; but 

 this is not yet proven. 



Dicti/oxi/hn Grievii. — This plant has many points of affinity 

 with I). Oldhamium ; nevertheless it has very distinct features of 

 its own. Its central or medullary axis is very large in proportion 

 to the thickness of its exogenous ring ; the former consists of 

 cellular parenchyma, throughout which are scattered numerous 

 bundles of exquisitely reticulated vessels unprovided with any 

 special sheaths. The largest vessels are nearest the centre of the 

 axis, the peripheral ones becoming smaller, more numerous, and 

 grouped in more continuous masses. Immediately surrounding 

 this A^asculo-cellular axis is a thin ring of similar vessels, but 

 arranged in radiating laminae, separated by well-defined medullary 



