OF THE FOSSIL PLANTS OF THE COAL-MEASURES. 
47 
the simplest possible type. They rarely consist of more than two, and often of but one, 
linear series of cells ; when the former, the two or more cells are rarely exactly vertical 
to each other ; blit, as shown in the drawing, the two are somewhat inclined, often in 
opposite directions, to the vertical line. I have not been able to detect any of these 
cells in the central triangle ; they appear to be confined to the exogenous layers. Of 
course I am aware of the old argument likely to be renewed against my nomenclature, 
and to hear the objection that there can be no medullary rays where there is no 
medulla ; but the obvious reply is that a medulla is not necessary. The rays in the 
thick, ligneous, pithless roots of an exogenous forest tree are the homologues of the 
similar structures in its stem, notwithstanding the entire absence of a pith from the 
former organ, just as they continue to be rays in the stem itself when some aged oak 
has become hollowed into a huge cylinder. 
The Leaves . — Having had the good fortune to discover a number of stems invested 
by a perfect outer bark, with well-preserved nodes at intervals of about half an 
inch, I at once commenced a vigilant search for leaves, and I was soon rewarded beyond 
my most sanguine expectations. Having made a vertical section through one stem 
which exhibited a perfect node, from each side of the thin lenticular margin of which 
there projected a slender leaf-like organ, as seen in Plate I. fig. 5 and Plate III. fig. 15, 
I proceeded cautiously to grind away the matrix external to the bark of the same specimen, 
in hope of discovering evidences of a verticillate arrangement. I soon came upon sections 
of four leaf-like organs, arranged in regular verticillate order. These sections were of the 
same type as those represented in Plate III. figs. 14 & 17. But proof was needed that 
these objects actually belonged to the node seen on the opposite side of the thin slab of 
stone. To obtain this, I continued to grind until 1 reached the margin of the disk, 
which presented itself in the form represented by Plate III. fig. 14, l l. The actual 
relation of the leaf-like organs to the disk was now thoroughly established. 
The sections of the individual leaves, as they appeared in the regularly disposed verticil 
before I ground away that portion of the section, were similar to those still seen at 
Plate III. fig. 14, m & mi, but in an inverted position. These were gradually traced 
backwards to the corresponding portions (III) of the same figure, in which we have a 
section of part of the margin of the foliar disk from which the leaves sprang, and the 
under surface of which has already become grooved at two points, defining the bases of 
the three leaves III. At m" we have a longitudinal section of one leaf from the side 
of the disk, whilst at mi mi we have the inverted tips of two of the leaf bases 1 1 , which 
had been bent inwards by the pressure of the earthy matrix, and consequently were divided 
a second time by the same section. At mi” is a section of another leaf, which, having 
been intersected obliquely, is longer than would be the case with an exactly transverse 
section. Plate III. fig. 15 is a longitudinal and slightly tangential section of the outer 
bark ( k ) of a stem passing through a node. Here we have a considerable portion of a 
leaf (m) intersected longitudinally. Its upper portion has been bent back upon the stem 
by external pressure, in the manner already referred to in my description of Plate Hi. 
