206 
PROFESSOR W. C. WILLIAMSON ON THE ORGANISATION 
original axile buildle widens out, as the stem grows stronger, into a tube, which is for 
the most part closed all round, and has only at each node, below the insertion of the 
leaf, a relatively small slit, or foliar gap, through which the medullary parenchyma is 
connected with the cortex, and from the margin of which one or several bundles pass 
into the leaf.”* I think there can be little question but that this widening out of an 
axile bundle which, as the same author observes, “ extends itself, and forms a tube, 
which surrounds a parenchymatous cylinder of pith” (de Bary, loc. cit., p. 283), 
presents substantially an analogous condition to what is so general amongst the 
Carboniferous Lycopods. At the same time, though the two cases are identical in 
their general features, they present differences. I have not yet found any Fern in 
which a solid central bundle develops a medulla wdthin its own component vessels. 
The medulla is existent ah initio, surrounded by a circle of such vessels. Thus, in 
Aspidium filix mas this medulla has a diameter of about a tenth of an inch close to 
the growing apex of the stem, and at its utmost development it rarely attains to more 
than four times that diameter; even then the interspaces between the bundles 
composing the entire vascular cylinder are very large. This is very different as to 
details from some examples found amongst the Lepidodendra, where a medulla 
developed from some invisible cell-germ has expanded to more than an inch in 
diameter. 
Plate 8, fig. 28, represents a transverse section of a Rachiopteids, of which I have 
several sections. At the first glance its central vascular bundle, cut through 
transversely, has a Zygopteroid aspect, but it differs in the fact that, whilst its side 
a consists of a line of large vessels, its opposite side h is composed of very small 
ones ; at the two points c, c' the bundle consists of a number of the small vessels 
resembling those at h, but here forming an irregular mass surrounding a vacant space 
at each end of the central portion, a, 6 ; at a cluster of these small vessels is detached 
in Zygopteroid fashion, as if going to supply some peripheral organ. The outer 
cortex is composed of an uniform cellular tissue. I propose the name of Rachiopteris 
irregularis for this very distinct organism, which is from Halifax. 
* English Translation, p. 284. 
