of the Fossil Plants of the genus Sigillaria. 131 
for a while to consider a point in the anatomy of this genus. It possesses 
a medullary sheath, in the shape of a circle of apparently isolated bundles, 
situated inwardly, but in proximity to the ligneous zone: the bundles 
are composed of elongated tubes, which, in their markings, are interme- 
diate to the true spiral vessels of Exogens, and the scalariform tissue of 
certain Endogens, such as Ferns. A medullary sheath of this kind ap- 
pears to be unknown amongst either recent or fossil plants, not even 
excepting the Zamias, to which Sigillaria possesses some affinity. 
The Monocotyledons, such as Palms, possess a number of apparently 
isolated bundles, which are generally very numerous, and disposed 
without much order throughout the diameter of the stem. The only 
plants of this class in which the bundles are less numerous, and more 
regularly distributed, are Ferns. These may be deferred for the present, 
as they will have to be referred to in another place. 
None of the longitudinal sections which Brongniart has given of 
Sigillaria elegans afford us an insight as to the longitudinal arrangement of 
the bundles of the medullary sheath. Now, in order to obtain this, we 
are compelled to examine the corresponding part of some allied plants : 
for example, the Coniferous Gymnosperms, and the Vascular Crypto- 
 gams.* 
By dividing in the longitudinal sense a young shoot of any ordinary 
Conifer, and extracting the pith, the woody cylinder is made to exhibit 
its inner side, which, by using a common magnifier, will be seen to be 
furnished with a number of very elongated oval-shaped openings, which 
are caused by the fibres being separated into bundles, alternately dis- 
uniting and approximating ; in fact, the inside of the cylinder has the 
appearance of net-work with very elongated meshes: and if the tissue 
of the entire cylinder be submitted to the microscope, it will be seen 
that there are two kinds, namely, spiral vessels and discigerous tubes, 
and that the latter form the whole of the cylinder, with the exception of 
its inner side, which principally consists of the former: it will also be 
seen that the openings or meshes are filled with parenchyma, and that 
they afford a passage for the bundles of fibrous tissue, which pass into 
the leaves. From these facts we have ascertained what is well known in 
the anatomy ofa Conifer, that the few inwardly situated spiral vessels con- 
stitute the medullary sheath, while the more numerous discigerous tubes 
form the ligneous system ; but we have also learnt that the meshes are 
the same as the vascular passages of Stigmaria and Anabathra, and that 
the medullary sheath is in the form of a netted cylinder. As regards 
the meshes, they need only be alluded to, because of their shewing that 
what has hitherto been considered a singular character in the first of 
ne ict 8 
* My inability to procure any portion of an American Zamia must be ace 
cepted as the reason why the Coniferous section of the Gymnosperms has only 
been examined. 
