482 PEOFESSOE W. C. WILLIAMSON ON THE ORGANIZATION 
specimen (figs. 9, 10) where the entire diameter of the branch, bark included, is only 
*2 of an inch, the distance from one canal (e) to another is from -03 to *04. In specimens 
ranging 1 from half an inch to an inch in diameter these distances increase from -08 to *1, 
whilst at the base of some very large arenaceous casts I have evidence that they increased 
to ‘25. These steady enlargements of the wedges accompanying the general growth of 
the stem demonstrate a corresponding intercalation of new vascular laminae into the exte- 
rior of each wedge as the exogenous development of the woody zone increased its diameter. 
Secondary Medullary Bays . — Whether viewed in their transverse or vertical section, 
we discover that the laminae composing each woody wedge are separated from each other 
by vertical cellular films, apparently undistinguishable by any important, definite feature 
from the medullary rays of the higher Exogenous Plants. Fig. 6 represents portions of 
four laminae (y) from a transverse section of a wedge. Two of these are in contact, as 
is not unfrequently the case ; the others are separated by lines of cellular tissue, which 
in most instances can be distinguished, in this section, by their darker hue, and by their 
cells possessing thinner and less sharply defined walls than is the case with the vascular 
tissues; but sometimes, in the transverse section of the stem, these cellular lines so 
closely resemble the vascular laminae, that they are scarcely capable of being distinguished 
from each other. Such distinction is easy enough in the vertical sections, whether tan- 
gential (fig. 5) or radial (fig. 11). In the former we see that these cells (fig. 5, d) are 
arranged linearly in single vertical series, there being rarely more than two vessels in 
direct contact with each other. Sometimes we observe one isolated cell; in others 
several cells are superimposed. In all cases they are compressed laterally, so that their 
length greatly exceeds their transverse diameter. Their transverse septa vary their 
direction indefinitely, being sometimes rectangular, in others oblique and overlapping. 
Plate XXIV. fig. 11 represents one of several sections made with the utmost possible 
care. The section is a radial one, passing through the longitudinal canal (e) at its inner 
margin, and through the exact centre of the woody wedge at its opposite or cortical end. 
This exactness was rendered necessary by the circumstance that the two lateral surfaces 
of each woody wedge are impressed by similar cells belonging to the primary medullary 
rays ; and it has been supposed by one of our authorities in Phytology that I had mis- 
taken the one for the other. There is not now the slightest room for such a supposition. 
In this section we see the cells of the pith at b. At e we have the longitudinal canal ; 
the entire range of the section to the left of the canal, in the drawing, consists of a 
lamina of barred vessels, whilst delicately projected upon the walls of the vessels 
throughout a great part of the section we have the vertically arranged cells of these 
secondary medullary rays. They exhibit the strongest tendency to arrange themselves 
murally and in continuous lines, stretching from the pith to the bark, only instead of 
representing modern bricks disposed horizontally, as is usual in the muriform medullary 
rays of Phanerogamous Exogens, they more resemble the long bricks of ancient Eome 
set upon their narrow ends. Still the extreme regularity of their arrangement justifies 
me in describing it as muriform. 
