OF THE FOSSIL PLANTS OF THE COAL-MEASURES. 
493 
In young stems and branches these fistular cavities coexisted with a surrounding pith, 
consequently when they became filled with sand or mud, the latter materials would harden 
into a permanent cast of each hollow cavity. But though such casts might exhibit the 
nodal constrictions due to the transverse medullary diaphragms, they would show none 
of the longitudinal ridges and furrows characteristic of Calamites. The reason for this 
would obviously be the intervention of the pith between its central cavities and the inner 
surfaces of the woody wedges to which such furrows were due. Hence no one, finding- 
such a cast, would recognize it as belonging to a Calamite. But as each stem increased 
in age and dimensions the pith gradually disappeared, not by mere decay, but by a vital 
process of rupture and absorption. As in the living Exogens, the existence of the pith 
seems to have been correlated with the first formation of the circle of woody wedges ; 
and when the growth of these was fairly started, it seems to have been no longer needful 
to the plant except at each node, where it continued to exist either as a complete or, 
what is more probable, as a partial diaphragm. After this disappearance of the pith, 
successive additions continued to be made to the exterior of each woody wedge, as in true 
Exogens, and also to that of the primary and secondary medullary rays. No definite traces 
of concentric rings indicative of interrupted growth appear in the transverse sections*. 
The pith does not appear to have been necessary to the plant whilst these external addi- 
tions were being made to its woody tissues. Fortunately one specimen has been found 
by Mr. Butterworth in which I have discovered remarkable evidence of what has taken 
place as the pith disappeared. Plate XXVII. fig. 26 represents three woody wedges (f) 
and two primary medullary rays (c, d) from a stem which must at least have had a dia- 
meter of 1 inch. The wedges and medullary rays present the usual features, but the pith 
is everywhere breaking up into large spaces, which, opposite the primary medullary rays, 
assume the definite rounded form represented at fig. 26, d, d. The shrivelled, half 
absorbed cells at V have nearly disappeared, and within this line they are wholly gone. 
At d, d the absorbent action has reached its limits ; it has touched the boundary line 
indicated by fig. 21, producing that undulating outline of the medullary cavity which 
gives to its common arenaceous casts their calamitean form, and which here, as elsewhere, 
is only found in stems of notable dimensions. In the specimen of Calamopiius, which 
I have previously referred to as described in the Transactions of the Manchester Literary 
and Philosophical Society, the absorbent action has gone yet further. Almost every 
trace of the pith has disappeared through its operation. The inorganic cast of the 
medullary cavity exhibits its sharply defined Calamitean outline in immediate contact 
with the persistent vascular and cellular tissues of the woody zone ; and it is an inter- 
esting fact that the specimen which thus exhibits the entire completion of the process 
of pith-absorption is the largest in which I have hitherto discovered structure, its 
* In this respect the Calamites only exhibit the same phenomena as appear in Dadoxylon, Dictyoxylon, 
and other exogenous stems of the Coal-measures, in British specimens of which I have rarely seen concentric 
rings of growth that I could identify with periodic checks arising from secular variations of climate. Never- 
theless such rings do occasionally occur. 
MDCCCLXXI. 3 Y 
