1873.] 



Plants of the Coal-measures. 



395 



reticulated type. "When divided transversely, it presents a triangular 

 section, the triangle having long narrow arms and very concave sides. 

 The bark is already differentiated into two layers, and has its exterior 

 deeply indented by three lateral grooves — one opposite to each concave 

 side of the vascular triangle. The outer layer is prosenchymatous, with 

 vertically elougated cells ; the inner one consists of cylindrical paren- 

 chyma arranged in radial lines, the cells being also elongated vertically. 

 As the plant grew, successive vascular layers were added exogenously to 

 the exterior of the vascular axis. Each layer consisted of a single linear 

 row of vessels, which were of large size opposite the concavities of the 

 triangle, and small where they approached its several angles. The radial 

 arrangement of those in the several growths was equally regular ; they 

 were disposed in single radiating series, new laminse being intercalated 

 peripherally as the stem grew. These radiating laminse were separated 

 by small medullary rays. Owing to the fact mentioned, that the laminae 

 radiating from the concave sides of the central triangle consisted of much 

 larger vessels than those radiating from its angles, three or four such 

 growths sufficed to convert its concave sides into slightly convex ones, 

 whilst a few more such additions converted the vascular axis into a 

 solid cylindrical rod. At this stage its transverse sections appeared de- 

 finitely divided into six radiating areas — three of large open vessels 

 radiating from the sides of the primary triangle, and three of small ones 

 proceeding from the sides and extremities of the angles. When these 

 growths have thus given a cylindrical form to the vascular axis, a change 

 takes place in its further development. Concentric growths again begin 

 to form, but in them all the vessels are of almost equally small dia- 

 meters : hence the abrupt termination of the three areas of large 

 vessels in the younger growths produces a distinct circular boundary 

 line, marking a special stage in the genesis of the stem. From this 

 point the additions go on uninterruptedly, the vessels of each radiating 

 lamina or wedge increasing slowly in size from within outwards as the 

 stem advances towards maturity. During these further developments the 

 bark has continued to be separated into two well-defined forms. An inner 

 layer consists of very delicate elongated cells with square ends (prismatic 

 parenchyma) ; these are seen in the transverse section arranged in ra- 

 diating lines proceeding from within outwards. The outer bark consists 

 of narrow, elongated, prosenchymatous cells, having very thick walls ; at 

 intervals, corresponding with the spaces between the successive verticils 

 of leaves in the ordinary examples of Asterophyllites, we find distinct nodes 

 where the bark expands into lenticular disks. The vascular axis passes 

 through these nodes without undergoing any visible change, either in the 

 position of its vascular layers or in giving off vessels to the nodes or their 

 appendages. The thin peripheral margin of each node sustains a verticil 

 of the slender leaves of Asterophyllites, of which there are about twenty- 

 six in each verticil. The aspect, dimensions, and arrangements of these 



