Most collections of fossil plants include specimens of tree-trunks or branches in which the bark is preserved in great perfection, while no trace of the structure of the trunk within the bark remains. Striking examples of this type of fossilization, in which the bark is preserved in exquisite detail while the space within the bark is wholly filled with fine siliceous sediments showing no trace of the original plant structure, occur frequently among the various species of Sigillaria and Lepidodendron. The fossilized armour-like outer cortex of Lepidodendron fails to retain any remnant of the inner woody material about as often as the various molluscan species of Spirifer fail to preserve their delicate internal spires. We know from the silicified specimens which have been found that the greater part of the trunk in Lepidodendron is occupied by a soft middle cortex which is very readily disposed of by micro-organisms. The relatively small cylinder of secondary wood which is found in most species which have attained to a large growth was itself rather susceptible to decay, the tracheids being relatively thin-walled, with the medullary cells very large and the rays voluminous. Innumerable examples of the entire failure of the woody tissue of these Lycopods to survive the processes which left the outer cortex admirably preserved occur in the fine-grained sandstones of Pottsville age in Orange County, Indiana.
Woody-tissue respiration