48 
PROFESSOR W. C. WILLIAMSON ON THE ORGANIZATION 
fig. 14. The somewhat clavate appearance of the free extremity of the leaf ( l ) is 
due to a slight accidental obliquity in the section at that point due to another flexure 
of the leaf carrying it out of the plane of the section. The thickness of this leaf ranges 
between *0043 and -0086. 
Besides the evidence afforded by the examples just described, I have obtained several 
transverse sections made more or less in the plane of the foliar disk, and which display 
the verticillate arrangement of the divergent leaves in the most definite manner. One 
of these is represented in Plate III. fig. 16 ; it will be observed that, in the expanded 
portion of the disk (F), the cells of the tissue evince somewhat more of a tendency to 
assume a radial linear arrangement than is usual in the outer bark. The bases of 
six linear leaves (m m) are seen radiating from the lower and left-hand sides of the section 
with considerable regularity. The breadth of each leaf is about '021. I discover no 
trace of vascular tissues, either in the leaves themselves or amongst the cellular tissues 
of the foliar disk from which they arise ; and it is a further curious fact that no 
vacant spaces exist, either in the outer (k) or middle (A) bark, such as are common 
amongst the Lepidodendra , where they were originally occupied by vessels that have 
disappeared. The transverse section of the central vascular axis, at this important 
point of the stem, exhibits its usual appearance ; and it is obvious that, if any of the 
very large vessels composing it had ever been deflected into the leaves, their large size 
must have made them conspicuous objects, whilst their divergence must have left some 
considerable gap in the regularly concentric and radial lines of vessels ; but no traces 
of such conditions have been discovered. The leaves appear as if they consisted wholly 
of cells, which, as we have already seen from Plate I. fig. 5, are mainly, if not entirely, 
derived from the outer bark *. Unfortunately these foliar tissues are much disorganized 
and rendered indistinct through carbonization, consequently the exact arrangement of 
their component elements has yet to be made out ; at the same time I have clear 
evidence that the central part of the leaf, at least, was chiefly composed of long, narrow, 
parallel-sided cells, and that the remainder was occupied by a somewhat coarse parenchyma. 
I have stated that the diameter of the bases of the leaves in Plate III. fig. 16 is - 05. 
This is exactly the breadth of the bases of the leaves in a very distinct verticil of 
Asteropliyllites which I possess in an ironstone nodule from the Lower Coal-measures of 
the Oldham district, and which I think belongs to the same species as the sections 
described. The leaves in the specimen referred to are more than three quarters of an 
inch in length, and with twenty-seven or twenty-eight in the verticil. In the example 
represented in Plate III. fig. 16, I calculate, from their angle of divergence, that there 
* Since the above description was written, Professor Renault has informed me that he can demonstrate, 
in one of his stems of Sphenopliyllum, eighteen vascular bundles given off from the central triangular axis, 
and passing outwards at a node to be distributed to six leaves, each of the latter receiving three primary 
bundles which pass outwards to three corresponding teeth at the extremity of the leaf. Of course this obser- 
vation renders it exceedingly probable that the apparent absence of small vascular bundles from my specimens is 
due to mineralization and is not a primary feature of the plant. See the note on p. 45. 
