72 
PROFESSOR W. C. WILLIAMSON ON THE ORGANIZATION 
size than those constituting the exogenous layers, occupying the same central position, 
and suggesting the idea of a primary vascular axis whose arrangements had been 
disturbed during the subsequent growth of the root. All these appearances received 
their explanation on my obtaining some additional sections, one of which is represented 
in Plate IX. fig. 57 ; I am indebted for this fine example to Mr. J. Whittaker, of 
Oldham. Both its general structure and its peculiar bark at once identify this as being 
identical with Plate VII. fig. 46. 
Plate IX. fig. 58 is an enlarged view of the central portion of fig. 57, in which we 
find the triangular axis of Aster opkyllites in its most definite form. The identity of this 
structure in the two conditions of stem and root, both in its contour and in the arrange- 
ment of its vessels, is most obvious. Its diameter, too, corresponds closely with that of 
the axes of the larger aerial stems ; the only difference that I can detect appears in the 
larger size of its vessels, compared, for instance, with those of Plate II. fig. 9. It will 
be observed also that the laminae of the young exogenous growth curve away from each 
of the angles of the central bundle in exactly the same fan-shaped manner as is seen 
in Asterophyllites — an arrangement to which I have already alluded when I said that 
the axis of Calamostachys Binneyana (Plate VI. fig. 38) exhibited some Asterophyllitean 
features in its exogenous layer. The vessels of the exogenous layers of the root-structures 
are much smaller than in the aerial stems described. This difference may be merely 
characteristic of a distinction between the roots and stems, or it may be due to the 
circumstance that the roots belong to a different species from those of which I have 
described the stems. The latter view is sustained by the fact that I possess several 
examples of undoubted amyeloid* roots having the structure seen in Plate IX. fig. 59. 
Some similar ones discovered since this figure was drawn are not only much larger and 
more perfect, but are invested by the characteristic bark of these amyeloid roots, re- 
moving all doubt as to the generic identity of the entire series of specimens. Plate IX. 
fig. 59 is drawn to the same scale as the corresponding sections of the aerial stems. We 
thus see that the vessels of the former correspond closely, both in size and arrangement, 
with what we see in Plate II. fig. 10, allowance being made for the non-existence in 
Plate IX. fig. 59 of the central triangle. The closeness of these general as well as 
detailed resemblances renders it probable that, whatever may be the case with Plate VII. 
fig. 46, Plate IX. fig. 59 belongs to the same species as Plate II. fig. 10 and the allied 
sections. We see in Plate IX. fig. 59 the same tendency to the formation of crescentic 
exogenous growths as appeared in Plate VII. fig. 46 & Plate IX. fig. 56. 
There are a few physiological features in the roots now described meriting attention. 
* As my investigations progressed, I have found it necessary to assign provisional generic names to certain 
well-marked types of structure on the same principle that the Lepidodendroid and Sigillarian roots were desig- 
nated Stigmariac before their real nature was known. Nor is this process wholly without a permanent use ; 
even after the true correlation of the objects with well-known and previously named plants is discovered, such 
names serve as convenient descriptive adjectives. Thus we now speak of sternbergian piths and diploxyloid 
stems. So the term amyeloid conveniently designates these pithless roots of the Coal-measures. 
