16 
PROFESSOR W. C. WILLIAMSON ON THE ORGANIZATION 
the cells of this meristem tissue are shown in fig. 31, <7. The outer extremities of all the 
medullary rays terminate in it ; and as they do so those of opposite sides of each primary 
wedge bend inwards more or less obviously towards an imaginary line prolonged through 
the centre of the wedge into the bark. Indications of this arrangement are seen at g' 
in fig. 32. 
The usual aspect of the masses of cortical parenchyma separating the primary vascular 
wedges, as they appear in vertical tangential sections, is shown in several of the figures, 
but especially in figs. 27, 30, & 35, A. In these sections it appears as ordinary paren- 
chyma, whilst in radial sections of the same tissue its cells exhibit a tendency to 
arrange themselves in semimural fashion, as seen at fig. 27, h 1 . 
External to the cortical structures just described is another and yet more extensive 
one, which appears to have been the primitive tissue of these stems. I infer this from 
the fact that in the young twigs (figs. 24 & 25) it is the only cortical tissue observable, 
except the epidermal layer (A), and occupies the entire area between that layer and 
the central vascular bundle ( c ). Its thickness relative to the size of the stem is also 
well seen in fig. 23. It consists of a coarse but thin-walled parenchyma, the cells of 
which are of unequal sizes, but generally about '005. The mean thickness of this 
layer in fig. 23 is about -025. It appears distinctly in figs. 32, 33, & 34, i, investing 
both the vascular axis of the primary stem and the branch which is being given off 
from the latter ; whilst in fig. 34, in which the vascular area of the branch is entirely 
separated from that of the main axis, it constitutes the chief portion of the separating 
layer. In the three sections just referred to, whilst we see the tissue in question 
enclosing the more internal layers of the bark and their associated vascular areas, it 
wall also be noticed that in passing over the cellular radii which separate the six large 
vascular wedges it dips inwards towards the centre of the stem. This arrangement is 
well seen in fig. 34, i 1 , i". 
Another of the characteristic features of this very remarkable plant is found in the 
distinct epidermal layer (Jc) with which it is shown to be invested in figs. 23, 24, & 25. 
A more highly magnified representation of the same tissue, as seen in the transverse 
sections, but enlarged 284 diameters, is seen in fig. 28. Viewed in this aspect it con- 
sists of two somewhat irregular rows of cells, which are very distinctly different from 
those of the bark (i) which they immediately invest. Those of the outer bark (i) have 
their walls of a dark colour and mottled aspect — a condition apparently due to the car- 
bonization of the cell-contents which characterized them when living. Those of the 
epidermis, on the other hand, are more clear and transparent, as if they had been 
originally devoid of all coloured protoplasm or other cell-contents. They are also 
smaller in size, averaging, in transverse sections, about *008, and retain their regular 
form much more perfectly' than the larger and softer cells underlying them. The 
distinction between these two tissues becomes yet more conspicuous when Ave turn to the 
longitudinal section, fig. 36. We here see that the epidermal layer (fig. 36, k) is separated 
from the cortical parenchyma by a sharply defined line, whilst its component cells are 
