OF THE FOSSIL PLANTS OF THE COAL-MEASURES. 
43 
position. One of the grooves (&") is, in this example, a double one, being divided by 
a central longitudinal ridge. This is a feature which often reappears in older branches. 
Plate I. fig. 2 represents another section from the same coal-seam, which may possibly 
belong to a different species from the last one ; at all events it differs somewhat both in 
size and form. The central vascular triangle (c) is much more robust, having a maxi- 
mum diameter of -05, the extreme diameter of the entire section, including the bark, 
being T. The largest vessels near the centre of the vascular triangle are about ‘009 in 
diameter, those of the angles being smaller, whilst each of the extreme tips of the 
latter terminates in a cluster of very minute vessels, which do not exceed -0006. 
External to the vessels of the central bundle, which, as in the last specimen, are not 
arranged in any linear order, is a single investing row of vessels (d) applied to each of 
the concave sides of the triangle, and arranged in a linear series ; of these, the largest 
vessels occupy the centre of each concave line, the rest becoming smaller as they 
approach the apex of each angle. This is the first of a remarkable series of exogenous 
layers which successively invest the central bundle, and convert the triangular outline 
of this vascular axis into a circular one. The double inner bark (g) is again wanting. The 
outer one exhibits the same aspect as in the previous example, only the three lateral 
furrows (Jd) are larger and more concave than in Plate I. fig. 1, or indeed than in 
any other specimen that I have seen. 
In Plate I. fig. 3 we have a transverse section of the vascular axis of another 
specimen, in which the exogenous zone ( d ) has received a second layer of vessels ; and it 
will now be observed that whilst each of these concentric growths, composed of one 
lineal series of vessels, maintains its distinct individuality, nearly every vessel of the outer 
circle is placed in a direct line with one belonging to the circle on the inner side of it, 
the two combining as parts of a radiating series. This double concentric and radial 
arrangement is yet better seen in Plate I. fig. 4, where a third circumferential growth 
has been added. The central triangular axis is now larger than in the primary twig 
(Plate I. fig. 1). The enlargement is partly due to an expansion of each of its component 
vessels, and partly to an addition to their number. The extreme diameter of the axis 
between point and point is *06, and that of its largest and more central vessels about 
*005. Between these larger vessels and the exogenous layers we now find a thin series 
of very small vessels, forming the peripheral portion of the triangular axis, to which 
structure they certainly belong. These appear to me to have been developed (I 
know not how) subsequently to the appearance of the first exogenous layer. Of this 
latter portion we have three growths, of which the central one is, in this instance, the 
smallest. The maximum thickness of this united triple series is now *0125 ; and, though 
at one point there is a slight irregularity in the arrangement of the vessels of the two 
outer rows, they maintain in general the radiating order already spoken of. Owing to the 
constant large size of the more central vessels of each row, the original triangle with con- 
cave sides is now being converted into a convex one, the greater portion of each of its 
lateral boundaries having become somewhat oblate. In this specimen we have the middle 
g 2 
