320 
PROFESSOR W. WILLIAMSON ON THE ORGANIZATION 
presents an example in which the medulla has had an unusually great diameter in 
proportion to that of the vascular zone. 
This exogenous zone is made up of a variable number of primary vascular wedges 
(figs. 1, b, 5, b), each one of which is composed of numerous radiating vascular 
laminae separated by medullary rays. The remarkable uniformity in their size and 
the regularity in the arrangement of these wedges gives to the transverse sections of 
the medulla the star-shaped outline already referred to. At their inner or medullary 
apex each of these wedges commences at a few vessels of somewhat larger size than 
those composing the rest of the vascular zone. These are observable in fig. 1, but 
they are much more conspicuous in some examples than in others. Fig. 7 represents 
a section, for which I am indebted to Mr. Bittterworth, in which these vessels are 
extremely conspicuous. The medulla of this specimen is much disorganized by 
mineralization, but it has not been fistular. Its diameter is much less than is usually 
the case, contrasting strongly in this respect with fig. 5. In Mr. Binney’s monograph 
on the Calami tes'”' he figured, at page 20, what he believed to be part of a primary 
wedge of a Calamite, adding the remark, that in these Calami tes “ the wedge-shaped 
bundles of pseudo -vascular tissue originate from a small circular orifice or opening, 
sometimes simple, as in the specimen now under consideration, but in other instances 
apparently divided into several parts, as shown in the annexed woodcut (fig. 3).” I 
have no doubt that the latter examples were specimens of Astromyelon, which, like 
myself, Mr. Binney then mistook for Calamites. 
The inner extremities of the primary woody wedges, where seen in transverse 
sections, have generally an obtusely convex outline rather than a sharply cuneate one. 
Fig. 5, however, represents an example in which they are much more wedge-shaped 
than usual, whilst in fig. 4, partly owing to the direction of the section, but yet more 
to a peculiarity in the specimen, the normal contour of these woody wedges is much 
less obvious than ordinary ; still traces of their prevailing aspect are seen at the 
lower side of the base of the lateral branch a . Each primary wedge is composed of a 
numerous series of laminae, each one of which has a mean thickness of about ’002. 
The vessels of each lamina are arranged with the utmost regularity in a single 
radiating series, reminding us, so far as the transverse sections are concerned, of what 
we see in the woody prosenchyma of living Conifers. On making a tangential section 
we see that these vascular laminae are separated by numerous medullary rays. Very 
frequently single laminae are thus separated from each other. In other instances we 
see as many as three such laminae unseparated by intervening rays. The vessels 
(fig. 6, b) rarely exhibit any special structure, but here and there, as at fig. 6, b', we 
discover proofs that they were of the usual barred type, and that where the transverse 
bars are not visible their absence is merely a result of mineralization. The ordinary 
medullary rays (fig. 6, c) are arranged in single vertical series. The cells are elongated 
vertically and much compressed laterally. Occasionally a ray consists of a single 
* ‘ Observations on tbe Structure of the Fossil Plants found in the Carboniferous Strata,’ 1868. 
