OF THE FOSSIL PLANTS OF THE COAL-MEASUEES. 7 
figs. 3 & 4, we find little more in each transverse section than a thin layer of a tissue (b) 
which is much more dense and opaque than the parenchyma which it encloses. Its 
inner margin is irregular. Longitudinal sections, like fig. 5, indicate that this tissue 
consists of compact prosenchyma, though in these specimens the mineralized condition 
is not favourable to the display of the details of its structure. In tangential sections 
of somewhat larger rachides we see this prosenchyma ( b ) is grouped in longitudinally 
disposed bands (fig. 15, b), which subdivide and again coalesce with each other at 
irregular intervals. These prosenchymatous cells appear to be slightly thickened by 
internal deposits, and have a diameter of from '00125 to '00062. In the above section 
the spaces between the prosenchymatous bands are occupied by prolongations of the 
medullary parenchyma (a). In the specimen, fig. 1, these fibrous bands are arranged 
in the transverse section as a series of small wedges, the contiguous bases of which 
form the outer boundary of the section, whilst their narrowed inner angles are 
separated by corresponding but inverted wedge-shaped prolongations of the medullary 
parenchyma. This arrangement is clearly shown by fig. 16, which represents a small 
portion of the cortical layer of fig. 1, further enlarged to 80 diameters. The coalesced 
bases of the prosenchymatous wedges are seen at b , b, and the outward prolongations 
of the medullary parenchyma ( a ) are seen at a', a'. Since this section is made from a 
petiole that was wholly detached from the outer matrix, I cannot be sure that it exhibits 
the entire peripheral portion of the cortical tissues. But whether it does so or not it will 
be noticed that the fibrous wedges are arranged with considerable regularity in one linear 
series, and that no detached prosenchymatous bundles exist within that linear series. 
On turning to fig. 17, which represents a similar section to the last, but taken from 
Captain Aitxen’s specimen, we see that we not only have, at b, the wedges similar to 
those in fig. 16, but in addition we have an inner series of detached rounded or elliptical 
bundles at b', V, and a further set of crescentic ones at b", b", the latter being in contact 
with the large gum-canals ( c , d) which abound in the cortical portion of the section. 
In the section (fig. 16) the cortical layer has a thickness, measuring from the base to 
the apex of each wedge, of about *016. The area occupied by the prosenchymatous 
bundles in fig. 17, amounting to '03, including the detached ones, will sometimes be 
rather greater than in fig. 16. In the specimens described thus far I find no definite 
trace of any layer external to the prosenchymatous one; but I have some examples in 
which I find clear evidence that in them the parenchymatous tissues are prolonged 
beyond the prosenchymatous ones, and appear to constitute a yet more peripheral layer 
of parenchyma. This is very clearly shown in sections made from the same specimen as 
that of which fig. 17 represents a portion. When this is the case the whole of the 
prosenchymatous bundles become enclosed in parenchyma, and converted into more or 
less completely detached hypodermic islets. This condition is somewhat important to 
us when we endeavour to ascertain the homologies of these structures in relation to 
living Ferns. 
Similar prosenchymatous bundles to those just described, but of smaller size, occur 
