102 
PROFESSOR W, C. WILLIAMSOR OR THE ORGANISATION 
paitJy tliroiigh the transverse enlargement of the entire stem. The result is that 
so long as these changes do not extend beyond the state represented by fig. 21, 
the exterior of the cast, a, neither can, nor does, become longitudinally ridged 
and furrowed. But in the specimen represented in fig. 22, all these conditions 
are altered. The entire diameter of this specimen is about ‘55 of an inch, v/hilst 
the maximum of its medullary cavity reaches ‘4. It will he observed that not 
only all the medullary cells have disappeared from this specimen, but even the 
larger and less dense cells composing the medullary extremities of the primary 
medullary rays, c, c, have also been absorbed ; the inner angles of the various trachemal 
woody wedges, h, h, have alike resisted pressure and absorption, and have thus 
given to the longitudinal ridges and furrows so characteristic of the exteriors of 
these inorganic casts their rounded undulating contours. This specimen thus 
answers a question that has been put to me by some of my correspondents, 
viz., “ If the ridges of an ordinary Calamite were formed in accordance with 
your hypothesis of internal casts, how is it that the ridges have a rounded 
outline instead of projecting as radiating, vertical plates, corresponding to the 
entire forms of the primary medullary rays to which you say those ridges owed 
their existence ?” The reply is, the process of absorption only reached the inner¬ 
most extremities of those primary rays. As the latter organs passed outwards 
through the xylem zone, their component cells became smaller and more capable 
alike of resisting pressure and decay. I have not seen a single instance in which 
these more external cells had been absorbed during the lifetime of the plant. The 
fistular cavity thus formed has ordinarily been filled with sand or mud, but in 
the present example, as is so frequently the case with the plants preserved in our 
Lancashire and Yorkshire nodules from the Gannister Coals, this cavity is occupied 
by calcareous matter, which has been in a state of solution, and which has filtered 
through the tissues of the plant. The first deposits from this solution have formed, 
in fig. 22, stalagmitic layers, a, a', lining the walls of the fistular cavity ; but in 
the central part, a, the intruded material has assumed a crystalline condition. 
These secondary details, however, in no w'ay affect the general conclusion, viz., 
that the ridges and furrows marking the exteriors of transverse sections of these 
internal casts exhibit rounded contours, and not stellate ones. The above specimen 
was obtained from above the Upper Foot Coal at Sholver Lane. My cabinet contains 
various other specimens which lead to the same conclusions. 
The section, part of which is represented in fig. 21, is one of a series of nine, 
made from the same stem or branch. Eight of these are transverse sections, which 
show a gradual increase of from 14 to 24-25 in the number of the tracheseal wedges 
of the xylem as we ascend from below. I am indebted to my old auxiliary, Mr. 
George Wild, not only for this series, but also for the section fig. 22. The specimen 
from which the latter section was cut was collected by another of my valued coad- 
iutors, Mr. James Nield, of Oldham. 
