290 
PROFESSOR W. C. WILLIAMSON ON THE ORGANISATION 
rays (fig. 4, h). A cambium layer obviously existed along the lines i, i, of figs. 1, 2, 
and 4. 
Externally to the xylem zone, B, we have what constitutes the most characteristic 
feature of this plant, viz., a true phloem zone, C. Each of the phloem-masses, 
Jc, corresponds in breadth to that of the xylem wedge to which it belongs. In 
many of these phloem-masses their component tissues are preserved only in a 
compressed or disturbed condition ; but in the figure, Plate 21, fig. 4, Jc, we discover 
transverse sections of cells and tubular structures of various sizes, which exhibit a 
strong tendency to arrange themselves, especially at the inner portion of the phloem, 
in radial lines. In this example the xylem portion of the bundle extends laterally 
from g to g. The phloem, Jc, extends upwards and outwards in this figure until its 
peripheral margin comes in contact with the inner one, p, of the true cortex. This 
illustration is enlarged 48 diameters. 
The large primary medullary rays which separate the xylem portions of contiguous 
fibro-vascular wedges or bundles now expand, as they proceed outwards, into magnificent 
primary phloem rays (Plate 21, fig. 1 ; Plate 22, fig. 2 ; and Plate 21, fig. 4 ; n, n ). 
d’he botanist will at once recognise the remarkable resemblance of these conspicuous 
phloem rays to those occupying a corresponding position in the phloem of the common 
Lime, which resemblance has led me to adopt the specific name of Tiliceoides for this 
species of Heterangium. Each of these rays, as seen in transverse sections of a 
stem or branch, fig. 4, n, becomes broadly trumpet-shaped as it proceeds outwards 
from the extremity of the medullary ray, g, of which it is an extension. The large, 
more or less cubical, parenchymatous cells composing it are arranged in irregularly 
curved lines, the concavities of which are directed towards the medullary axis. Ex¬ 
ternally these cells merge with those of the inner cortex, p. Secondary and smaller 
phloem rays, extensions of the secondary medullary rays, Ji, can also be traced in all 
the transverse sections. They are sufficiently distinct in the phloem, Jc, of the isolated 
wedge, c, of fig. 5. 
The cortex, D, is separable into two layers, an inner one, p, and an outer one, r. 
The inner one (figs. 1, 2, and 4, p, p ) consists of a comparatively uniform parenchyma 
which presents no special features of interest. Tire outer cortex, r, r, transverse 
sections of which are seen in figs. 1, 2, and 6, is somewhat more complicated. It 
consists of parenchymatous cells of variable forms and sizes. As seen in figs. 2 and 6, 
the inner cells, r, are much larger than the outer ones, v, r. The orifices, w, w, of 
fig. 6 ate merely worm-borings and not normal cavities. Two important additional 
structures exist in transverse sections of the outer cortex. One of these consists of 
transverse sections of ascending vascular bundles grouped in pairs. In Plate 21, fig. 1, 
two imperfect examples of these twin arrangements are seen at u and u; two such 
bundles are enlarged 5l4r times in Plate 22, fig. 7, u, u, and a similar pair of still 
larger ones is enlarged 20 diameters at u , u, of fig. 8. 
The second structure seen in transverse sections of the outer bark consists of 
