OF THE FOSSIL PLANTS OF THE COAL-MEASURES. 
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in the decay that has overtaken the cellular structures as that they have become loosened 
from their attachments by that decay, and floated out when water reached them. Be 
this as it may, it is the bast-layer, with its investment of thick -walled epidermal cells, 
which has furnished, in nearly every case, the carbonaceous film that covers the stems of 
the Lepidodendroid plants so abundant in the shales and sandstones of the Coal-mea- 
sures. The differences so obvious between the aspect of the outer surface of the thin 
film of coal and that of the subjacent shale are too well known to require further 
reference. When the carbonaceous matter is detached, the specimens are spoken of as 
being decorticated ; and there maybe no objection to the retention of a convenient term 
provided we distinctly understand the sense in which it is used. In all such instances 
the entire woody and inner cortical structures equally disappeared. The part which 
remained was, as I have already pointed out, the epidermal layer, with the semifibrous 
portion of the prosenchymatous one, which I have invariably found in every specimen 
that I have examined in which the structure is preserved. This bast-layer evidently gave 
to the bark the faculty of resisting the decay which so effectually cleared out all the more 
central tissues. It was this double layer which constituted the cylinder, the two sides 
of which were brought together and flattened by superimposed pressure when the stems 
were prostrated, and which constituted the hollow mould into which mud and sand were 
poured when they remained erect. We thus learn that very large trees were flattened 
into a thin layer, not because their stems were succulent, but because these hard woody 
and cellular cortical tissues broke up or were floated out of their epidermal sheath ; 
whilst the latter, though strong and tough, was sufficiently flexible to yield to the super- 
incumbent pressure, often without any material degree of disturbance of its integrity 
through fractures. Hence the fine flat masses of Sigillaria and Lepidodendron not 
unfrequently met with under the conditions which I have described. 
It is a remarkable circumstance that whilst the woody zone is the part that has so 
frequently disappeared amongst the larger specimens of Lepidodendroid plants, and 
especially amongst the Sigillarise, it is the part which is the most frequently preserved in 
the Stigmarian roots of the latter plant. I presume that this fact is to be explained by 
the different circumstances surrounding the two structures. The stems overthrown by 
storms were equally exposed to the decomposing influences of a warm humid atmosphere, 
whether they were prostrated on the ground or stood up as decapitated stumps. Such 
atmospheric influences would speedily destroy all but the tough superficial layers. The 
roots, on the other hand, imbedded deeply in wet mud, would be preserved from all 
atmospheric action ; hence the beautiful preservation of their vascular tissues : these 
are often compressed and displaced, but rarely destroyed. The cellular bark, on the 
other hand, with the exception of the epidermal layer, and also the medullary cells, 
have yielded much more frequently to the decomposing influences that surrounded them 
even though protected by the soil. 
What we know of the origin of the leaf-scars in living plants has left little room for 
hesitation respecting their nature in the fossils under consideration ; but some of the 
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