288 
PROFESSOR W. C. WILLIAMSON ON THE ORGANIZATION 
Ill my memoir, Part III., Plate 43, fig. 19, I represented a section of a young 
dichotomising branch of the Burntisland Lepidodendron, in its young state, in which 
the vascular cylinder had divided into two horseshoe-shaped portions, and in fig. 20 of 
the same plate is shown a similar subdivision of the same plant in the more advanced 
Diploxyloid stage of growth. Fig. 8 represents a young branch of Lepidodendron 
selaginoides dividing in a similar manner. Each of the two halves of the vascular 
axis exhibits the usual horseshoe-shaped contour, the innermost cortical cells, b", 
intervening between the two becoming contiguous to and intermingled with the 
medullary ones. The whole of the cortical elements and foliar-vascular bundles seen 
in this specimen are identical with those represented in figs. 5 and 6 ; but whilst at 
the upper side of the half, A, of the vascular axis we have the crescentic beginnings 
of an exogenous zone, h, no trace of such a zone is seen in the half B. Thus the latter 
is Lepidodendroid and the former Sigillarian according to the Brongniartian hypothesis. 
Immediately surrounding the lower half of each axis, especially of B, we find some cells, 
b , arranged in radiating lines, but no trace of vessels. This disposition of cells, occupy¬ 
ing the position of a cambium layer, so to arrange themselves is not unimportant. 
Having thus traced the history of an example of M. Renault's group a, from its 
Lepidodendroid to its Sigillarian state, I will now deal with his group b. 
The specimen of Lepidodendron Harcourtii described by Witham and Brong- 
niart was made the foundation of a restored figure by the latter botanist in his 
‘Vegetaux Fossiles.’* The imperfection of the specimen necessarily led to imper¬ 
fections in the details of the restoration. 
M. Renault, speaking of Lepidodendron Harcourtii as the typical representative 
of his second group, says,-—-“On peut s’etonner qu’avec un developpement semblable 
aucune trace de production ligneuse exogene ne se soit pas encore manifestee en dehors 
du cercle de faisceaux vasculaires (Toil partent les cordons foliaires.” And again, “ On 
peut done legitimement conclure que la difference dans le diametre des rameaux de 
Lepidodendron n’apporte pas de changements dans la disposition generale des tissus 
que Ton trouve dans ces plantes.” On both these points I shall offer evidence that 
our English specimens exhibit the conditions of which M. Penault denies the 
existence. 
Fig. 9 represents a section of a superb example of the young state of Lepidodendron 
Harcourtii, for which I am indebted to Mr. Aitken. On comparing it with the 
similar section of L. selaginoides it is seen to differ from that plant in two points : 
first, in the absence of all vessels from the area occupied by the medullary cells, causing 
the inner boundary of the vascular medullary zone (“ etui medullaire” of Brongniart) 
to be sharply defined ; and, second, in the much greater uniformity in the size and 
the mineral charcoal, so abundant in almost every British coal of Carboniferous age, is largely composed of 
small cubical fragments of prosenchymatous bark. Fig. 7, f, illustrates the strong tendency of this 
tissue to be broken up into similar fragments. 
* Plate 21, fig. 4. 
