OF THE FOSSIL PLANTS OF THE COAL-MEASURES. 
401 
If we now turn to bundle 2 and interpret it by the light afforded by the preceding 
description, we shall see that it is about to undergo similar changes. The bundle 
itself exhibits little change of position in the several sections, but the prosenchyma- 
tous bark has altered its form so that the bundle is in nearly the same condition 
in fig. 44 that bundle 1 exhibits in figs. 38, 39, Sc 40. We here learn several 
important facts. The first is that whilst the vascular bundles retain the same un- 
changed positions throughout a considerable length of the stem, they each in turn 
move outwards and disappear at the periphery of the bark: the second is that the 
irregularities in the outline of the bark are not wholly due to external pressure, but to 
the successive development of a series of peripheral appendages, the bases of which have 
risen from the bark in the form of sharp, projecting, longitudinally disposed ridges with 
broad bases. During the progress of these changes we always find that, immediately 
external to each vascular bundle, the parenchymatous layer of the bark consists of a 
large mass of very regular and translucent parenchymatous cells, as represented in 
Plate XXIX/ fig. 35, h", and an expansion of which tissue has evidently constituted the 
great bulk of the appendicular organ, whatever it was, since similar masses of paren- 
chyma retain the same position in the enclosed areas external to bundle 5 in 
Plate XXX. figs. 41 & 42. I do not doubt that all the lenticular areas of regular 
parenchyma (Plate XXX. figs. 37, h', Sc 41, h') enclosed within lines of more condensed 
cellular tissue were destined in turn to receive new bundles of vessels at points higher 
up in the stem. 
I have no difficulty in determining whence these bundles were derived. I have called 
attention to the coalesced clusters of small vessels which occupy the peripheral portion 
of the vasculo-cellular medullary axis immediately within the exogenous layer. Small 
masses of these clusters, consisting of vessels intermingled with medullary cells, first 
become isolated, and then tend outwards, emerging through the exogenous zone, as in 
Plate XXVIII. fig. 30, m'", the exogenous radiating laminae becoming separated to allow 
of their escape. On the opposite side of the figure just quoted, we observe a break in the 
continuity of the exogenous ring, from which vacant space a similar bundle has 
obviously emerged. Nearly every transverse section in my cabinet exhibits similar 
evidences that the bundles originated as I have just described. Before considering the 
probable significance of these growths, there remains to be noticed another peculiar, 
cylindrical, divergent appendage of a different kind, and which is represented in 
Plate XXX. fig. 36, n. It consists of a mass of cells and vessels, or rather pseudo-vessels, 
both equally reticulated, which appear to take their rise in the cellular portion of the vas- 
culo-cellular medullary axis. I have not been able to trace any of the true vessels of that 
axis within the diverging appendage, though I cannot affirm that there is no connexion 
between the two tissues. At its base this large bundle has been about *25 in diameter, 
but it soon contracts to T2. The section having cut through it somewhat obliquely, 
its more central portion is seen in the specimen figured, and its peripheral extremity 
appears in a contiguous section made from the same specimen. By combining the two 
mdccclxxiii. 3 H 
