OF THE FOSSIL PLANTS OF THE COAL-MEASURES. 
289 
structure of the smaller cells composing the various layers of the bark. The foliar- 
vascular bundles also much more frequently retain their barred vessels undestroyed. 
They appear in the sections as dark, dense bands, instead of being represented by 
vacant spaces. 
No example of L. Harcourtii in which all tissues of the stem are preserved has 
hitherto been described. The mean diameter of the branch figured is about lij? inches. 
The cellular medidla, a, is invested by the vascular medullary zone a, which, in turn, 
is enclosed within a dense layer of inner bark b, the innermost portion of which 
is crowded with foliar bundles. Surrounding this is the middle bark cl, composed of 
small cells. This layer, which is so rarely preserved in L. selaginoides, is as 
rarely absent from L. Harcourtii. The cells of the innermost portion, e, of the outer 
bark are, as is usual in the Lepiclodendra, somewhat larger, and have thicker walls, 
than those of the inner bark, whilst the prosenchymatous zone, f appears as a conti-' 
nuous, wavy, but very narrow zone, curving slightly outwards opposite to the base of 
each leaf, and projected outwards as a sharply defined funnel-shaped prolongation, 
wherever one of the numerous foliar bundles, c, passes through it. The leaves, g, 
differ in no material respect from those of L. selaginoides. 
Fig. 10 represents part of a transverse section of a branch of rather larger size and 
somewhat more developed growth.'" It exhibits the usual structure of the vascular 
and cellular portions of the medullary axis as well as of the inner bark. The central 
portion of the cellular medulla, a, has been accidentally destroyed. The vascular 
medullary cylinder, a, is composed of barred vessels which diminish in size from 
within outwards. Its peripheral margin presents numerous radiating points, c', which 
are the bases of as many foliar-vascular bundles. These bend upwards immediately 
after their orientation, as in L. selaginoides ; hence they are intersected transversely 
at c, c, where they severally occupy the numerous crescentic areas intervening between 
the projecting points c. The cells of the parenchyma of the innermost portion, b, of the 
inner bark are very minute, usually forming a dense sheath surrounding each foliar 
bundle c. In the middle portion of this inner bark is an irregular layer of cells, b'. of 
somewhat larger size; but its outer zone again consists of smaller cells, b". In this 
outer zone a narrow band of these small cells appear arranged in radiating lines, b"'. 
This radiating zone bears an important relation to some features to be described. At 
cl we have the thick middle bark with its dense foliar bundles, c", c". 
Even in this typical Lepidodendron, when the branches are sufficiently advanced in 
growth, there appears a rudimentary zone composed of true radiating, exogenously 
developed, vascular wedges. Fig. 11 represents a segment of a transverse section of a 
portion of the zone corresponding to b", b"' of fig. 10, enlarged 27 diameters. At b'" 
are the radiating lines of cells already referred to, and at b" the outermost part of the 
* It will be observed tbat before it begins to develop its exogenous zone this plant attains to much 
larger dimensions than tbe L. selaginoides does. In this respect it corresponds closely with the Arran 
plant. 
