OF THE FOSSIL PLANTS OF THE COAL-MEASUKES. 
399 
further difference is seen in the entire absence from D. Grievii of all trace of the thin 
outermost or subepidermal layer of parenchyma which occurs in young shoots of 
1). Oldhamium. 
Vascular bundles . — I have already alluded to the existence in the bark of large 
bundles of vascular tissue, which ascend vertically through the stem. In transverse 
sections these bundles exhibit a more or less oval section (Plate XXVIII. fig. 30, m rnJ 
& m", & Plate XXIX. fig. 35, m & ml), the longer axes of which are parallel to radial 
sections, or to the circumference, of the bark. Most of these bundles exhibit some 
tendency towards a division into two lateral halves, either in a median constriction of their 
peripheral outline (Plate XXIX. fig. 35, m'), in a disposition of the component vessels to 
group themselves round two determinate centres (Plate XXVIII. fig. 30, to'), or in an 
actual line of demarcation dividing the bundle into two parts. These bundles chiefly 
consist of barred vessels, with some reticulated ones, imbedded in a mass of small cells 
identical in size, colour, and general aspect with those of the medullary axis. Under a 
high magnifier these cells can generally be traced between the two divisions of the 
vascular bundle. No part of this investigation has caused me such serious trouble as that 
relating to these bundles. It was easy to see that they most probably originated somehow 
in the vascular axis, and that they proceeded outwards towards the periphery to supply 
leaves or fronds. The difficulty was to obtain proof of all this, owing to the circum- 
stance that a large number of longitudinal sections threw little light upon the matter. 
The bundles were present in the sections ; but, owing to their small size and want of 
perfect straightness in their course, I could not trace that course over any considerable 
area ; whilst, so far as I was able to do so, I obtained but few and faint evidences of 
divergence from parallelism with the central vascular axis. At last, having obtained a 
favourable specimen, nearly two inches in length, I made, at measured intervals, the 
series of eight transverse sections represented in Plate XXX. figs. 37-44. These sections 
gave me the key to the history of this portion of the plant. 
We sometimes find these bundles inside the innermost cylinder of bark-cells, as in 
Plate XXVIII. fig. 30, m, and partially in Plate XXIX. fig. 35, m. In the latter instance 
the bundle is almost imbedded in this bark-tissue, a condition not unfrequently met with. 
They further occur in all parts of the middle parenchymatous layer, but never in the pro- 
senchymatous one. So far as I can ascertain, the number of these bundles, in a perfectly 
free state, in any one transverse section is usually seven or eight, the latter being the 
largest number that I have yet seen. In the sections represented on Plate XXX. we 
find only six; but the specimen was imperfect at one side, and one or two bundles 
doubtless disappeared in the missing portion. Of these sections, I believe fig. 37 to. 
belong to the lower, and fig. 44 to the upper extremity of the fragment ; at all events 
such has been the case if, as I expect, the bundles have supplied leaves and not roots. 
In the figures of these sections I have not filled in the details of their organization, 
but the outlines of the various structures have been drawn to the same scale and with 
the utmost attention to accuracy of measurement. In each of them a represents the 
