1871.] 



Calamites of the Coal-measures. 



269 



as the woody tissues increased exogenously, these cellular tracts also ex- 

 tended outwards. Radial longitudinal sections exhibited in these the same 

 mural tissue that occurs in the woody wedges. Hence the author gives to 

 the former the name of primary medullary rays, and to the latter that of 

 secondary ones. The structure of the medullary and ligneous zones is 

 compared with that of the stem of a true Exogen of the first year, of which 

 transitional form Calamites may be regarded as a permanent representative. 

 Tangential sections of this woody zone exhibit parallel bands of alternating 

 vascular and cellular tissue, running from node to node. At the latter 

 points each vascular band dichotomizes, its divergent halves meeting cor- 

 responding ones from contiguous wedges, and each two unite to form one of 

 the corresponding bands or wedges of the next adjoining internode. 



The Barky hitherto undescribed, consists of a thick layer of cellular 

 parenchyma, undivided into separate laminae, and not exhibiting any special 

 differentiation of parts. This structure exhibits no signs of external ridges 

 or furrows, being apparently smooth. The stem was enlarged at each node, 

 but the swelling was less due to any increased thickness of the bark at these 

 points, than to an expansion of the woody layer at these points, both ex- 

 ternally and internally. This was the result of the intercalation of nume- 

 rous short vessels, which arched across each node, their concavities being 

 directed inwards, and which constituted the portion of the woody zone 

 that encroached upon the constricted pith at these nodes. Several modi- 

 fications of the above type have been met with, most of which may have 

 had a specific value. In one form no canals exist at the inner angles 

 of the woody wedges ; in another, laminae, like those of the woody 

 wedges, are developed in the more external portions of the primary me- 

 dullary rays, those occupying the centre of each ray being the most ex- 

 ternal and latest formed. The primary ray is thus transformed into a 

 series of secondary ones. 



In another type the vascular laminae of each woody wedge are few in 

 number, and the component vessels are the same ; but the latter are re- 

 markable for their large size. In a fourth variety, the exterior of the 

 woody zone has been almost smooth, instead of exhibiting the usual ridges 

 and furrows : this variety is also remarkable for the large size of its me- 

 dullary cells, compared with that of the cells and vessels of the woody 

 zone. 



But the most curious modification is seen in a plant previously described 

 by the author under the name of Calamopitus, in which round or oblong 

 canals are given off from the medullary cavity, and pass horizontally 

 through each primary medullary ray of the woody zone to the bark. These, 

 being arranged in regular verticils below each node, are designated the 

 infranodal canals. The verticils of small round or oblong scars, seen at 

 one extremity of the internodes of some Catamites, are the results of this 

 peculiar organization. In one species of this Calamopitus y instead of the 

 longitudinal canals of the woody wedges terminating at the nodes, they 



