OF THE FOSSIL PLANTS OF THE COAL-MEASURES. 
287 
numerous transverse sections of tlie foliar vascular bundles, c , their more internal 
portions, c, passing almost horizontally through the exogenous zone.* 
When my memoir, Part II., was written, I was unacquainted with any further 
advance in the exogenous development of this stem than is seen in the specimens just 
described ; but it is otherwise now. The cabinets of Mr. Aitken and Mr. Cash have 
furnished me with several specimens displaying the same stem in various stages of 
further Diploxyloid growth. Fig. 7 represents one for which I am indebted to 
Mr. Aitken, enlarged nearly six diameters. The central vasculo-cellular medullary 
axis has a mean diameter of about '25.+ In another specimen in Mr. Aitken’s cabinet 
the diameter of this axis is fully '4, which is also the diameter of the same structure 
in a similar section for which I am indebted to Mr. Cash. In a series of specimens in 
my cabinet, the entire stems of which exhibit a gradual increase in size, I find that in 
the youngest, in which there is no exogenous growth, the diameter of the vasculo- 
medullary axis is '02. In the next, in which the exogenous zone has completed two- 
thirds of the circle, it is '05. In another, in which the exogenous zone has completed 
its circle, it is - 1. In fig. 7 it is '25, whilst in Mr. Aitken’s and Mr. Cash’s specimens 
it has attained to '4. We thus find that this central axis steadily increases in magni¬ 
tude with the general growth and age of the branch; and an examination of the 
specimens shows further that this increase is not due to a mere enlargement of the 
cells and vessels of which this vasculo-medullary axis is primarily composed, but to an 
enormous, though gradual, increase in their number. It thus becomes evident that, 
contrary to the views of M. PlENAULT, the cells of the axis retained their genetic 
activity long after the exogenous zone was developed, and consequently the latter 
must have been capable of such interstitial changes as allowed it to expand and 
accommodate itself to the increasing diameter of the structure which it enclosed. 
The exogenous zone, h, of fig. 7 is composed of very regular, narrow, radiating wedges, 
undistinguishable, on a casual glance at the transverse section, from the similar struc¬ 
ture in a Gymnospermous branch. The medullary rays, Tc, are numerous and large, 
and nearly all of them contain more or less numerous barred cells that are elongated in 
the direction of the ray. The comparatively narrow cortical zone e is the same as the 
zone e of fig. 1, whilst the prosenchymatous zone f is now the dominant one, having 
more than four times the diameter of <?, as well as being four times thicker in propor¬ 
tion than the same zone is even in fig. 4. I have frequently had occasion to point out 
the remarkable tendency displayed, both by the Calamitean and Lepidodendroid plants 
of the Coal-measures, to the extension of this prosenchymatous zone of the bark, which 
has here played the part of a periderm. 1 
* In illustration of these peculiarities of direction, see figs. 33 and 34 of my Memoir IX. The vertical 
direction which they follow, close to the exterior of the vascular medullary axis in the non-exogenous 
state, is equally transferred to the exterior of the exogenous cylinder when that structure first makes its 
appearance and when, by age, it has attained to large dimensions. 
t These measurements are in fractional parts of an inch. 
X This portion of the bark in fig. 7 is not devoid of further interest- 1 long ago ascertained that 
