126 Contributions towards Establishing the General Character 
filled with mineral matter. Such is a brief outline of the internal struc- 
ture of Stigmaria. Ona general comparison with some recent plants, 
especially the Cactuses, this fossil does not offer much disparity. In 
these we have a similar broad zone (the bark), a similar hollow cylinder 
of wedge-shaped bundles (the wood), and a similar central part (the 
pith) ; but these are all the points of agreement, as the ligneous system 
of the Cactuses does not consist of a uniform tissue, nor are the walls of 
this tissue marked with lines, as in Stigmaria (Brongniart). Sigillaria 
elegans, too, somewhat resembles this plant in the general aspect ofa trans- 
verse section. The characters of the tissues in both, it will be seen, are in 
agreement, but Stigmaria is entirely divested of the circle of apparently 
isolated bundles which lies within the ligneous eylinder of Sigillaria, 
Fragments of Stigmaria are often found having the same outline as 
that of the specimen which has been figured: like this, they have their 
pith, bark, and radiating interspaces, occupied with mineral matter ; but 
instead of there being any remains of tissue in the wedge-shaped bundles 
of the cylinder, there are nothing but vacant spaces. The difference 
between these two kinds of specimens has evidently been caused in the 
following manner: in the kind first described, the soft cellular tissue 
composing the pith, the bark, and the radiating interspaces or plates, as 
they may be more conveniently termed, rotted out, simultaneously with 
its being replaced by mechanically induced mineral matter in the shape 
of mud, which hardened soon after its deposition ; on the contrary, the 
tissue of the cylinder, owing to its firmer texture, resisted decomposition : 
it remained fixed in its original place by means of the outer zone, the cen- 
tral part, and the radiating plates, and, through some cause or other, the 
whole of the tubes and their delicate markings became mineralised, or 
electrotyped as it were; and thus we have preserved one of the most in- 
teresting objects of microscopic investigation. The other specimens were 
subject to the same changes up to a certain point—to the consolidation of 
the mechanical deposit ; but after this had taken place, instead of the 
tubes becoming electrotyped, they rotted out like the cellular tissue ; nor 
was their place afterwards filled up with any mineral matter, either 
chemical or mechanical: hence the vacant spaces which now remain. 
It is fortunate that we have the remains of Stigmaria in the state last 
described, since they enable us to investigate a doubtful point in the 
internal structure of this fossil. 
Brongniart, in his explanation of the sections illustrative of Stigmaria, 
which are added to his “Observations on the Internal Structure of 
Sigillaria elegans,” speaks of the radiating plates or “spaces,” as ‘ cor- 
responding to the great medullary rays.” Lindley and Hutton, in the 
« Fossil Flora,” (Vol. iii. p. 48), write to the same effect. Let us for 
a moment stop to inquire into the nature of medullary rays. According 
to Professor Lindley, ‘‘ they are composed of muriform cellular tissue, 
often not consisting of more than a single layer of cellules ; but sometimes, 
as in Aristolochias, the number of layers is very considerable.” ‘‘ No vas- 
