OF THE FOSSIL PLANTS OF THE COAL-MEASURES. 
217 
of ordinary but very delicate parenchyma, which gradually thins out, upwards and down- 
wards, into a single interrupted row of cells. This latter part connects these larger 
medullary rays with a multitude of smaller or secondary ones (f) seen in the same 
section. Sometimes these consist only of one single cell : more frequently we see 
two or more arranged in a single vertical series ; and from time to time still larger ones 
occur with two or even more parallel vertical rows of these cells, thus approximating 
their arrangement to that of the primary rays. The cell-walls of these secondary 
medullary rays are so exceedingly delicate and thin that it is not easy to trace them 
through the radial longitudinal sections of the ligneous axis; nevertheless careful 
manipulation of the light enables the observer to do so. At their inner or medullary 
extremity all these rays, primary and secondary, take their rise in, or rather are merely 
prolongations of the cellular medulla, the parenchymatous cells of the latter (Plate XXX. 
figs. 43-47, a) being unaltered in shape or arrangement in the immediate neighbourhood 
of these radial prolongations of the pith ; but on entering the medullary ray they soon 
become mural, being elongated in the direction of the ray. This elongation is seen 
equally in transverse sections (fig. 47 ,f) and in radial ones (Plate XXX. fig. 43 & Plate 
XXIX.'fig. 44, f). At their outer extremities (fig. 44, </') they merge in a corresponding 
manner in a delicate cellular tissue (fig. 44, </), which constitutes the innermost layer of 
the bark. 
We see in the transverse sections of the woody cylinder very clear evidences of suc- 
cessive and interrupted exogenous growths. At each of these lines the continuity of 
the radiating lines of vessels becomes wholly interrupted and a new series commences. 
These new vessels are at first very small and irregularly disposed, but, as we proceed 
outwards, they soon resume their regular arrangement and size. In one of my sections 
I have clear evidence of a new circle of these small and irregularly disposed vessels 
forming externally to the entire cylinder, as if in a cambium layer. These additional 
layers are not always added equally to the entire circumference of the Stigmaria ; they 
sometimes only surround some two thirds of that circumference, as is not unusual 
amongst living Exogens. But the most remarkable feature of the woody zone is 
supplied by the vascular bundles given off to the rootlets, and which reach them exclu- 
sively through the primary medullary rays. On making a tangential section of any 
portion of the woody cylinder, we discover appearances which are virtually repetitions of 
what is seen in Plate XXX. fig. 48, the latter being merely the external surface of the 
ligneous zone, which exhibits the same arrangement of tissues in all sections made parallel 
to that surface. The bundles of vessels (c), which are really the external surfaces of the 
radiating woody wedges seen in the transverse sections, alternately separate and reunite, 
leaving the large lenticular areas (f) constituting the primary medullary rays already 
described. As one of these rays proceeds outwards, the vessels bounding its sides at 
the upper angle detach themselves from the wedges to which they severally belong, 
and combine to form what, in the tangential sections, appears as a tongue-like projection 
hanging down (Plate XXIX. fig. 45, n, & Plate XXX. fig. 48, n) into the ray. A radial 
