On Different States of Fossil Vegetables. 101 
the stem of a peculiar genus, and the medullary cylinder 
which this vascular cylinder surrounds, presents peculiar 
transverse furrows, which, according to this author, have been 
made to characterise the genus Artisia. I may add that, in 
specimens of this stem, or of a closely analogous species from 
the mines of Saarbruck, I found an intermediate zone between 
the external surface and vascular axis, which appeared to 
‘ correspond to the origin of the bases of the leaves, and which 
affords all the characters of the stem figured by M. de Stern- 
berg, under the name of Knorria Sellowit. 
In stems with tissues imperfectly preserved, we must 
therefore carefully distinguish the different zones of tissue 
in the same stem, and their external and internal surfaces 
which produce so many different appearances. 
What I have said of stems applies equally to fruits, in 
which the thickness of the pericarp often gives rise to two 
very different forms, and in which the cavities, in other in- 
stances, are not real cavities, but, on the contrary, spaces 
occupied by a different destroyed tissue, and even sometimes 
by all the solid parts. 
Carbonised vegetables, or such as have passed into the 
state of lignites, give rise to fewer observations ; yet it must 
be remarked that, in this alteration, their tissues have often 
undergone modifications which render it difficult to under- 
stand them rightly. Lastly, it not unfrequently happens, 
that a portion of the organs of vegetables passed into the 
state of lignites, is transformed into pyrites, or else pyrites 
of a globular shape are formed in the middle of this tissue, 
and may, at first sight, be taken for a character of organisa- 
tion. The section of certain dicotyledonous fossil woods 
often in that case resembles a monocotyledonous stem. 
Petrifaction often gives rise to apparent changes in the 
tissues, the origin of which must be carefully attended to. 
1. Incertain cases all the tissues are not equally preserved 
during petrifaction ; and it is particularly in silicified woods 
that we see frequent examples of this. Most frequently the 
soft tissues, more easily altered, are destroyed, as in macera- 
tion, while the stem being placed in circumstances suited to 
silicification, and the more resisting tissues, have preserved 
their character while becoming silicified. Often in such a 
