226 Royal Society. 



their unusual length, and in'consisting of two, or even three, vertical 

 series of cells instead of one, as is usual with the secondary rays. 

 The vessels pursue their longitudinal course across the node unde- 

 flected in any direction, save where they bend aside to allow the passage 

 outwards of vascular bundles going off to the aerial branches*, as 

 represented in my figures 13 and 38. Thus in the exterior parts of 

 these large stems the ligneous zone exhibits little or no indication of the 

 presence of a node, except what these divergent bundles afford. I 

 find that these bundles slightly increase in size as they proceed from 

 within outwards, showing that they share in the exogenous additions 

 made to the exterior of the ligneous zone ; in one of my stems that 

 zone has a circumference of seven inches, and in the other of six 

 and three quarters. It is in the former one that I find the nodal 

 bundles ; but I have not seen one of these organs whose actual dia- 

 meter exceeds three sixteenths of an inch, confirming my previous 

 statements respecting the comparatively small size of the aerial 

 branches. As in my previously described examples, these bases of 

 branches exhibit no separation of the vessels into a circle of wedges 

 like those of the parent stem. The persistent growth of the vascular 

 bundles just described seems to indicate more permanent relations 

 between them and the central stem than I once thought probable. 

 There appears to be a close approximation to uniformity in the 

 number of the woody wedges of these large stems ; one of mine 

 contains 85, and the other 83 such. Mr. Binney counted 73 in his 

 large specimen (loc. cit. pi. 2. fig. 1). In the thin, young woody 

 cylinder represented in my fig. 1 9, the mean diameter of which was 

 slightly over an inch, the number was also about 80. This close 

 resemblance between stems so different in age and size again illus- 

 trates another of my previous statements, viz. that age produces no 

 increase in the number of the woody wedges, but that each one of 

 the latter enlarges by successive additions to its peripheral portions 

 of new laminae, which latter partly fill up the increasing area of the 

 enlarging circle, and partly encroach upon the primary medullary 

 rays, as represented in my figure 1 7, in addition to some interstitial 

 growth. 



We thus learn that as the ligneous cylinder of a Calamite in- 

 creased in age and size it gradually exhibited less and less of the 

 Calamitean peculiarities seen in young stems ; its external portions 

 assumed a generalized, unsulcated form, which recurs with remarkable 

 uniformity in several otherwise different plants of the Coal-measures. 



Amongst the Burntisland fossils sent to me by Mr. Grieve I 

 find two very curious stems, probably of the same general nature 

 as Zygopteris. Both have a dense outer cortical layer, with vascular 

 bundles in the interior. In the simpler of these plants the transverse 

 section of this bundle is crescentic ; but in the concave border of the 

 crescent are two small projecting capes dividing it into three minor bays 

 (fig. 2). In the other the vascular axis is a double one, lodged in a 

 somewhat elliptical stem : one of these is a simple crescent, the con- 



* Tliis condition is very correctly represented in plate 3. fig. 3 of Mr. Binney's 

 memoir on Calamites (Palteont. Soc.). 



