1 
460 PROFESSOR W. C. WILLIAMSON ON THE ORGANIZATION 
a, we have a somewhat confused admixture of cells and vessels, constituting a central 
axis, surrounded by an inner cortical zone, b, of radially compressed cells, arranged in 
an approach to regular circles, and passing outwards into a less regularly disposed series 
of much larger cells, c. From these latter, isolated lines of cells, cl, radiate, chiefly 
in single rows, like the spokes of a wheel. The nave of this wheel is composed of 
a thick zone, e, of large and irregular parenchymatous cells. The drawing is enlarged 
70 diameters—the maximum diameter of the section being about ’08 of an inch. 
Fig. 2 represents the transverse section described by Messrs. Cash and Hick at 
York, enlarged 55 diameters. It presents very different features from those of fig. 1 
so far as its central portion is concerned. At a we have a large medulla, about ‘04 of 
an inch in diameter, composed of very regular parenchyma, and closely resembling 
that of the Oldham Astromyelons. The central cells are much larger than the peri¬ 
pheral ones, some of them being ’005 in diameter. This medulla is surrounded by 
narrow, imperfectly developed, and somewhat irregular zone of vessels, b, arranged in 
radiating lines. The concentric disposition of the cells of the innermost bark, c, is well 
shown in this section, as are the radiating lines of cells, d, connecting this inner bark 
with the peripheral parenchyma e. Though these lines of cells have been somewhat 
disarranged by external pressure, we have no difficulty in tracing a continuous zone of 
them from d' to cl". 
Fig. 3 represents a very important section for which I am indebted to Mr. Spencer: 
important because it demonstrates the identity of my Astromyelon and the Myrio- 
phylloides of Cash and Hicks in the most decisive manner. In my description of the 
former plant I directed attention to some features very characteristic of it. The chief 
of these was the distinct outline and peculiar form of the conspicuous vascular wedges 
constituting the exogenous zone ( loc. cit., Plate 19, fig. 1, b), and the relations of these 
wedges to the large medulla which that zone enclosed. The lam hue of each wedge, b, 
converge at their medullary extremity, where there exists small groups of what, in my 
previous memoir, I designated “ a few vessels of somewhat larger size than those com¬ 
posing the rest of the vascular zone” (loc. cit., p. 320). I cannot quite satisfy myself 
whether these are actually vessels or liquified cells.* In either case they bear a definite 
relationship to the vascular wedges with which each of the several small clusters of 
them is associated. In fig. 3 the characteristic medulla is conspicuous at a. The 
clusters of cells or vessels just referred to are seen at a, and the distinctive forms ol 
the vascular wedges are not difficult to trace, as is shown at b. The exogenous zone 
has undergone a much greater development in this specimen than in fig. 2. The 
identity of the plant with the Myriophylloides of Cash and Hicks is shown by the 
retention of a small portion of the cortex. The innermost layer is imperfectly retained 
at c, but the radiating cellular laminae are sufficiently distinct at cl, terminating in the 
outer layer of cortical cells at e. 
* I Lave since obtained absolute proof that tliese are really vessels which may be regarded as con¬ 
stituting a medullary sheath.—July 6th, 1883. 
