OF THE FOSSIL PLANTS OF THE COAL-MEASURES.—PART XIII. 
293 
enter so largely into the composition of this stem. In the twin bundles (Plate 22, 
figs. 7-8), going off to what, both in this plant and the allied Heterangium Grievii, 
I presume to have been petiolate leaves, the vessels are mainly of the barred or spiral 
type represented on Plate 22, fig. 14. In the other vascular organs they appear, as 
represented in fig. 15, to be reticulated ; but more careful examinations, aided by 
higher microscopic powers, show that all these vessels are furnished with the bordered 
pits seen in Plate 21, fig. 16, the canals of which are all narrow, oblique, and parallel to 
one another. In the larger proportion of these vessels and tracheids the canals alone 
remain visible, as in Plate 22, fig. 17 ; but in fig. 18 we see the transition from the 
one form to the other in the same traclieid or vessel. At e we have in each pit both 
the central canal and its investing areole ; but at e' all the areoles have disappeared, 
I presume during mineralisation, the oblique canals alone remaining; but that each 
canal was originally surrounded by its own areole is unquestionable. 
One more curious feature is presented by this plant. In Plate 22, fig. 5, we have 
the exogenous xylem zone at B in its normal position relatively to the medullary 
axis A. But the two fibro-vascular bundles or wedges c, c', are being pushed out¬ 
wards by a mass of parenchyma, 2, which has developed on the inner side of the two 
bundles separating them from the medullary axis. 
It will further be noticed that each of these bundles carries away with it, on its 
inner side, a cluster of the medullary vessels b, b, suggesting the probability that the 
cellular mass, 2, has really been an outgrowth from the medullary cells, since it must 
have originated from the central side of the two clusters of medullary vessels b, b, 
which it has been instrumental in pushing outwards. I discover no clue to the desti¬ 
nation of these displaced fibro-vascular bundles ; but they remind us of somewhat 
similar conditions seen in the stems of Lyginodendron Oldhamium. 
Fig. 19 represents a cluster of cortical cells, enlarged 187 diameters. Cells in this 
condition are extremely abundant in the cortical layers. At the first glance they look 
like cells with thickened cell-walls, but they are constantly found in close associa¬ 
tion with cells belonging to the same parts of the plant, the walls of which exhibit 
no such thickening, hence the effect is possibly due to some condition of mineralisa¬ 
tion which has produced deposits of various amounts within the cell-walls. 
The next plant requiring further notice is the Kaloxylon Hookeri, which I first 
described in my Memoir, Part VIIA The specimens described and figured on Plates 5, 
6, and 7 of that memoir were all obtained from one of the Oldham localities, and, as 
is not seldom the case with the fossil plants from that quarter, their cellular structures 
are ill-defined through defective mineralisation. But for several years past we have 
obtained examples of a Kaloxylon from the Halifax beds which are in a very different 
condition. These specimens long ago showed that the cortical zone possessed 
peculiarities of structure which the Oldham exauqjles had not revealed to me; but 
on re-examining those examples, guided by the light thrown upon them by those 
* ‘ Phil. Trans.,’ vol. 166, 1876. 
