200 
PROFESSOR W. C. WILLIAMSON ON THE ORGANIZATION 
made from the writings of M. Brongniart points out, an exogenous mode of growth, 
a conclusion fully borne out by facts yet to be mentioned ; small cells arranged in single 
or double vertical rows pass outwards, like medullary rays, between these vessels. The 
tissue immediately surrounding the ligneous zone has almost always disappeared from 
the specimens of this plant, its place being represented by an almost vacant space ; but 
there are indications, as Mr. Carruthers has correctly pointed out, that it has been a 
delicate form of parenchyma. In the present example almost every trace has disap- 
peared save a narrow ring (g) of disorganized carbonaceous matter at some little distance 
from the ligneous zone. The space within this tissue represents the innermost portion 
of what I regard as the cortical layer. Scattered through this vacant space, as well as 
the more external one, we find in the transverse sections small bundles of minute scala- 
riform vessels (fig. 1, m) fringed round with delicate parenchyma, and exhibiting a 
circular or oval section. Their diameter ranges from •003 in the round sections near 
the ligneous zone to a longer axis of '007 in the more peripheral portions. 
We now come to the middle bark ( h ), a dense, well-preserved layer of thick- walled 
parenchyma, gradually passing into prosenchyma at its outer margin. The rounded cells 
of the former have a mean diameter of about *003, gradually becoming more oblong, 
with a longer axis of about ’007 and a shorter one of -002. The foliar vascular bundles 
make their way through this layer, the delicate parenchyma with which each bundle of 
vessels is surrounded gradually merging with its coarser cells. The delicacy of this paren- 
chyma investing the bundles frequently leads to its entire disappearance, leaving blank 
spaces (m!) channelled through the bark, in the middle of which the barred vessels of 
the foliar bundles are sometimes conspicuous from their isolation. As the parenchyma 
of this middle bark becomes converted into prosenchyma at its outer portion, its cells 
become elongated vertically, and at last pass rapidly into the almost vascular form of 
prosenchyma (Jc), constituting the bast-layer of the outer bark. In the transverse 
section these tubes are seen to be arranged in radiating series proceeding from within 
outwards. In the vertical section the more external ones become as elongated as in 
the pleurenchyma of many exogenous barks, the fibres being arranged longitudinally 
in curving lines having a very regular parallelism. They have a mean diameter of 
about "00083. Towards the outermost portion of this tubular prosenchyma we find, 
in these fossils, a tendency to split vertically, and to the consequent detachment of the 
epidermal layer (l). The innermost portion of this detached layer consists of tubes pre- 
cisely similar in every respect to those of the outer bark, but which again change rapidly, 
as we proceed outwards, first into the prosenchymatous form seen in the middle bark, 
and then into a thick-walled parenchyma which constitutes both the superficial portion 
of the epidermis and the entire substance of the petioles or bases of the leaves (l). I 
have here referred the tubular bast-layer partly to the outer bark and partly to the 
epiderm, because, when the latter becomes detached, the line of separation usually 
passes through the middle of the layer; but it may perhaps be more correct to regard 
the whole of these bast-tissues as one subepidermal layer. Fig. 5 exhibits a tangential 
