or THE EOSSIL PLAHTS OF THE COAL-MEASURES. 
(11 9 
olo 
they foreshadow the cambial growths of a later age. But another question arises, viz. 
What is the genetic relation subsisting between the innermost cellular layer of the bark 
of a Lepidodendron, through the agency of which these new exogenous growths have 
been developed, and the cambium ring, which has accomplished a similar end in living 
Exogens 1 
I have already stated that the entire cortical layer of the fossil Lycopods corresponds 
much more closely with that of the recent ones than it does with that of any living 
Exogens. We find in it nothing identical with the endophlceum or liber of English 
botanists, the Phloem of German ones. It is essentially a meristem tissue, the result 
of successive cell-fissions, and usually divisible into from three to four layers. We have, 
first, an outer parenchyma, which I have termed subepidermal, within which is a variously 
modified prosenchymatous layer. This is succeeded by an inner parenchyma, the inner- 
most portion of which is usually more or less differentiated into a reproductive layer in 
which the successive exogenous zones are developed. I have got some magnificent speci- 
mens of bark which show that the two outer layers, viz. the subepidermal parenchyma and 
the prosenchymatous one, but especially the latter, increased in thickness through a meri- 
stem action which from time to time developed an abundance of new cells along the line 
of separation between the two tissues, and which process is illustrated by the curious 
specimens of bark described and figured in my second memoir (Phil. Trans. 1871, p. 220, 
Plate xxxi. figs. 54 & 57). The growth of these two outer layers being thus apparently 
accounted for, we have further to ascertain the corresponding process in the history of 
the inner parenchyma. That it also increases enormously in thickness with the increased 
age of the stem is quite certain ; but though I have examined it in numerous specimens, 
I have wholly failed to detect any trustworthy traces of a diffused cell-fission or meristem 
process acting simultaneously throughout the entire substance of the layer. Such facts as 
I have observed seem to me to render it more probable that the new cell-divisions have 
taken place near its inner surface, and that, whilst these divisions were ultimately 
instrumental in adding to the thickness of the vascular ligneous zone on their inner 
side, they also increased the diameter of the parenchymatous bark-layer to which they 
belonged in the opposite direction. If this idea proves to be correct, it will follow that 
this meristem action of the innermost bark ends in the production of two kinds of 
permanent tissue — an inner vascular one, belonging to the vascular axis, and an outer 
cellular one, belonging to the true bark. 
These meristem processes have evidently taken place interruptedly. There seem 
to have been periods of intense activity alternating with periods of rest. The latter 
state is illustrated by specimens of Stigmaria in my cabinet like that represented in 
Plate xxxi. fig. 52 of my second memoir*, where the outer parenchyma (l) passes suddenly 
and abruptly into the prosenchymatous layer (Jc), the peculiar meristem structures seen 
in figs. 54 (h) and 57 of the same Plate being entirely wanting. But these latter evidences 
of vigorous action are very conspicuous in other specimens which I have discovered since 
* Philosophical Transactions, 1872, Part I. 
