130 PROGRESS OF MICROSCOPICAL SCIENCE. 



of tlie tliree 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 sepa- 

 rated into two well-defined forms. An inner layer consists of very 

 delicate elongated cells vnth square ends (prismatic parenchyma^ ; 

 these are seen in the transverse section arranged in radiating lines 

 l^roceeding from within outwards. The outer bark consists of narrow, 

 elongated, prosenchymatous cells, having very thick walls ; at inter- 

 vals, corresiDonding with the spaces between the successive verticils 

 of leaves in the ordinary examples of Aster ophyllites, 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 perijiheral margin 

 of each node sustains a verticil of the slender leaves oi Asterophyllites, 

 of which there are about twenty-six in each verticil. The aspect, 

 dimensions, and arrangements of these leaves correspond exactly with 

 what is seen in the ordinary specimens found in the Coal shales. 

 Transverse sections of them exhibit a single thick central midrib, but 

 no traces of vascular tissues have hitherto been found in them. 



The laminas of the vascular axis are separated by numerous medul- 

 lary rays of small size ; these rarely exhibit more than four or five 

 cells in any vertical series, and usually but one or two. The exterior 

 of the bark is deeply indented in each internode by three very deep 

 superficial grooves, each one of which occupies the side of the stem 

 corresponding with a concavity of the central triangle of the vascular 

 axis. These grooves, which are sometimes double instead of single, 

 extend from node to node, but do not indent the nodal disks. Owing 

 to the great depth to which these penetrate tlie bark, they give a very 

 characteristic tripartite aspect to each transverse section of these stems. 



The Burntisland type agrees with the Lancashire one in all its 

 leading features of structure and growth ; but its vessels are all 

 barred instead of being reticulated, and the author has not met with 

 such beautiful examples of its nodal disks as he has done in the case 

 of the other form, neither has he seen its leaves attached. On the 

 other hand, he has found specimens of much larger diameter than any 

 that have hitherto been detected in Lancashire, exhibiting the charac- 

 teristic peculiarities already referred to in an exquisitely beautiful 

 manner. The auihor has also obtained one section from this locality 

 in which a branch is given off. The vessels of this divergent organ 

 are derived from the central portion of one of the segments of small 

 vessels, seen in the transverse sections, which proceed from one of the 

 angles of the central triangle. 



Having elucidated the details of the aerial stems, the author pro- 

 ceeds to examine such organs of fructification as api)ear to belong to 

 these plants, commencing with the Volhnannia Dawsoni, which he 

 described at length in the Transactions of the Philosophical Society 



