324 
PROFESSOR W. WILLIAMSON ON THE ORGANIZATION 
vertical line varying from one-sixth to one-tenth of an inch in breadth, in which the 
vessels, instead of pursuing them regular course, are twisted and contorted, often 
dichotomising and interlacing in the most extraordinary manner, the irregular inter- 
spaces being occupied by expansions of the secondary medullary rays. As I described 
in my first memoir, this state of things is common enough in tangential sections where 
a branch is about to be given off, as in figs. 26 and 28 ; but I am wholly unable to 
explain its meaning here. Fig. 17 represents a small portion of the section referred 
to, in which d indicates the medullary rays and </ the contorted vessels. 
On turning 1 to fig. 15, which is a vertical section made through the centre of the 
specimen, we find that the section has most fortunately passed through a node, 
the cellular extension of the medulla forming the nodal medullary diaphragm being 
seen at n. It will be observed that there is no nodal constriction at the exterior of the 
vascular cylinder, f like that which is seen in the pith, and which forms so conspicuous a 
feature in the common sandstone casts of the medullary cavity. The most valuable, 
because, in my experience, unique feature of this specimen is seen in its bark, h. 
This is broken up into fragments, which, though often detached, and intermingled 
with Stigmarian rootlets that have penetrated between these fragments, nevertheless 
retain substantially their normal positions. A limited layer of parenchyma exists 
immediately external to the vascular cylinder, but the greater portion of the cortex 
consists of prismatic prosenchyma, the cells of which are arranged in regular radiating 
fines, an arrangement best seen in the transverse sections, as represented in fig. 18, 
which is enlarged 26 diameters. The cells have a transverse diameter of about , 0023. 
Their length varies greatly, but, as is the case with the cells of the primary and 
secondary medullary rays of the vascular zone, we find long radial fines of them having 
exactly the same length, each series terminating abruptly by coming in contact with 
other similar series composed of cells of a different but equally uniform length. 
Tangential sections made through this tissue exhibit the appearance represented 
in fig. 20. The cells here have almost always the oblique or acute extremities of 
prosen chymatous tissue. Fig. 21 represents a small portion of a tangential section, 
enlarged 10 diameters, in which a lenticular mass of parenchyma appears. This 
tissue seems to me identical in character with the similar masses of cells seen in the 
vascular zone, and which, as I have already shown, are the outward prolongations of 
the primary medullary rays. It will be curious if future observations should prove 
that these cellular structures are prolonged through the prismatic layer of the bark to 
its periphery. Yet I strongly suspect that such has been the case. 
1 do not suppose that this layer of prismatic prosenchyma constitutes the 
outermost bark. The specimen when found by Mr. N ield was worn and weathered, 
hence we have no means of knowing what was the entire thickness of the bark ; the 
probability is that there would be an outer parenchyma in which the genetic fine 
producing the prosenchyma would be found. This, however, is a mere probability 
which requires verifying. One thing is clear, viz., that the bark, as we see it in 
