68 
PROFESSOR W. C. WILLIAMSON ON THE ORGANIZATION 
numerous.” “ They may prove to be rhizomes and roots of the Asterophyllite described 
in my last letter to you.” 
I think I have now accumulated most satisfactory evidence that the above hypothesis 
is correct, and am able to prove that Amyelon is the root and rootlets of the stems 
described in the preceding pages. I would observe at the outset that the first exami- 
nation of the former plant would scarcely lead a naturalist to the above conclusion, 
since it exhibits no appearance of joints or special nodes, much less of verticillate arrange- 
ments of any kind, such as are seen in the rhizomes of Calamites. On the other hand, 
it is branched like the roots of an ordinary exogenous tree. Its bark also differs from 
that of Asteropliyllites ; but I think that I shall have no difficulty in tracing the homo- 
logous relations of the two conditions. In describing these specimens I will commence 
with the most characteristic example that I have seen, one of a series of valuable sections 
which I obtained from a nodule for which I am indebted to Mr. Nield of Oldham. Plate 
VII. fig. 46 is a transverse section of a young root surrounded by a cluster of branching 
rootlets, which have been chiefly, if not wholly, given off from two points in the section. 
The central axis ( d ) consists of a series of reticulated vessels of small size, the largest of 
them not exceeding -0018. These are arranged, in the usual exogenous manner, in very 
regular radiating laminae or wedges. The centre, whence these laminae spring, exhibits 
no trace of a medulla, nor, in the present section , of any other special tissue. In 
other sections to be described important differences present themselves in this respect. 
The vascular axis in fig. 46 consists of a large central mass of vessels, partly enclosed 
in a second crescentic layer representing a secondary exogenous growth. The latter 
portion, it will be observed, does not entirely enclose the central one — a condition to 
which I have referred in previous memoirs as not being uncommon amongst the Crypto- 
gamic plants of the Coal-measures. 
This vascular axis is surrounded by a bark consisting of two very distinct layers. 
The peculiar aspects of this tissue are better seen in Plate VIII. fig. 47, which is 
enlarged eighty diameters. Its inner layer (g) is an ordinary parenchyma, consisting of 
cells of very unequal sizes. Many of these are very small, but others are -0038, these 
larger ones being especially predominant towards the exterior of the layer. Many of these 
external cells ( g ') are subdivided tangentially to the circumference of the stem by a 
number of secondary cell-divisions. This parenchyma passes rapidly, but not abruptly, 
into the outer layer ( h ), in which cells, compressed tangentially, are arranged in irregular 
columns perpendicular to the surface of the structure. The boundary lines between 
contiguous columns are strongly marked, usually more so than the transverse septa of 
their component cells. The individual columns sometimes extend from the inner bark 
to the periphery ; at others they terminate at various points midway, where they 
become intercalated between other similar ones. 
Plate VIII. fig. 48 represents a radial longitudinal section of part of the root, a portion 
of a similar one being still further enlarged in Plate VIII. fig. 49. In the former figure 
the vessels of the axis ( d ) are seen to be crossed by numerous small medullary rays (f). 
