[ l'J5 J 
V. On the Organisation of the Fossil Plants of the Coal-Measures .—Part XVT. 
By W. C. Williamson, LL.D., F.R.S., Professor of Botany in the Owens College, 
Manchester. 
Received Marcli 5,—Read March 14, 1889. 
[Plates 5-8.] 
During the last twenty years many single examples of vegetable forms from Carboni¬ 
ferous rocks have come into my possession, which were obviously different from 
any hitherto described. But I have carefully abstained fi’om publishing such 
specimens until examples of each multiplied in my cabinet, enabling me to determine 
how far their apparently distinctive features were constant, and not merely individual, 
variations. Many such imperfectly kno'^vn forms still occupy a drawer in my cabinet ; 
but in the present Memoir I propose to describe several of which examples have 
accumulated so far as to enable me to speak with reasonable certainty as to their 
specific distinctiveness. 
In several of mj? previous memoirs I have from time to time called attention to a 
curious development of a medulla in the centre of the axial vascular bundle, especially 
of the Lepidodendra. This was especially done in the Memoir, Part III., when 
describing the Burntisland Lcpidodendron, to which, as was also the case with the 
Arran form (Part X.), I have not yet ventured to give a specific name. 
In the case of the Burntisland plant I showed, in figs. 3, 4, 5, 8, and 11, a medulla, 
a, which, at first of very minute dimensions, gradually enlarged, pari g)assu with the 
increase not only in the diameter of, but also in the number of the vessels composing 
the non-exogenous vascular cylinder—the “ etui medullaire ” of Brongniart. In this 
example traces of primordial medullary cells, however minute and few in number, 
could be detected in the youngest twigs. 
The Arran plant (Memoir, Part X.) presented different features. The very young 
leafy twigs, found in great numbers in the Laggan Bay deposit, had an axial vascular 
bundle, which consisted wholly of tracheids, in the interior of which no traces of 
cellular parenchyma could be found [loc. cit., figs. 1 and 2), whilst at a more advanced 
stage of growth such a medulla began to make its appearance {loc. cit., fig. 3), 
which ultimately attained to a considerable size {loc. cit., figs. 6, d, and 6a, d). 
Though unsuccessful in my search for an example in which the earliest traces of such 
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