44 
PROFESSOR W. C. WILLIAMSON ON THE ORGANIZATION 
bark (li) preserved. It consists of a very delicate parenchyma, the cells of which have a 
diameter of about ’0015. These are somewhat cubical, and are arranged in linear 
series, which radiate from the exogenous layers of the wood to the outer bark. This 
middle cortex does not appear to merge gradually with the outer one, but is separated 
from it by a somewhat sharply defined line. The outer bark (k) exhibits the same 
features that were seen in the previous specimens. The thick-walled cells are not 
arranged in linear series, as in the middle bark, but as in ordinary irregular parenchyma. 
In this example the three external internodal grooves ( k ’) are all double ones, the only 
instance in which I have found this to be the case. 
Before proceeding to trace the further development of these stems, it will be con- 
venient to examine the aspect which longitudinal sections of them present at this stage 
of their growth. This is shown in Plate I. fig. 5, which represents a section made 
through the centre of the triangular axis, and crossing one node and parts of two inter- 
nodes. The vessels of the central triangle (c) are undistinguishable in this section, save 
by their central position, from those of the exogenous layers, d d. They are all of the 
reticulated type, seen more highly magnified in Plate I. fig. 6. I have satisfied my- 
self that these reticulations are more distinct on the two sides of each vessel parallel 
with the direction of the medullary rays than on the sides at right angles to the latter. 
They are due to modifications of spiral deposits of lignine, and must not be confounded, 
as they have often been by Witham and some of his successors, with the disks of so- 
called glandular or discigerous fibres of the Conifera. 
Though the delicate middle bark (h) is rarely well preserved, I have in my cabinet 
sufficient examples of it to show that it appears, in longitudinal sections, in the form of 
long narrow prismatic cells (Plate I. fig. 7), with nearly, if not absolutely, square 
ends. They vary considerably both in length and in diameter. They are usually from 
•012 to *006 long, the former much more frequently than the latter, the shorter and 
broader ones occurring near the nodes. One of my sections shows traces of the thin layer 
of parenchyma (g) within this prismatic layer. The outer bark (fig. 5, k) is now seen to 
consist of long, narrow, thick-walled prosenchyma. I have found it difficult to ascertain 
the greatest length of the component cells, but I have traced them as far as from -03 to 
*035 : as is the case with the cells of the middle bark, they become shorter in the 
neighbourhood of each node (&'). At this latter point the bark swells out rapidly into a 
broad lenticular disk, from the thin margins of which verticils of long narrow leaflets 
{1) extend in very regular order. At these points the cells of the outer bark assume a 
very peculiar arrangement. Those which ascend from the internode below the nodal 
disk (Plate I. fig. 5, k") proceed outwards in a very definite series, forming the lower 
portion of the disk, whilst those which descend in slightly curving lines ( k ') from the 
internode above terminate rather abruptly against the curving lines of those of the 
lower series. These cellular tissues are prolonged uninterruptedly into the leaves, 
which I shall describe more fully by-and-by. In several of my specimens I have 
noticed that, a little below each node, a portion of the cells of the middle bark curve 
