OF THE FOSSIL PLANTS OF THE 00 A L-ME A STIFFS. 
71 
been one single cell. It is only by some such explanation as this that I can reconcile the 
appearances of the two sections just referred to with those of a tangential section made 
through the same structures. The latter section, as I have already stated, merely 
exhibits a very ordinary thick-walled parenchyma (Plate IX. fig. 55), affording no 
indication of the peculiarities seen in the sections represented in Plate VIII. figs. 47 & 49. 
The robust strength of the cell-walls of fig. 55 is altogether different from the more 
delicate transverse divisions of fig. 47, h, making it probable that the genesis of the two 
had been different. It was difficult to believe that these radial and tangential sections 
belonged to the same structures. By making an obliquely radial section through the 
tissue, I obtained the result seen in Plate IX. fig. 54, which I think demonstrates the 
accuracy of my interpretation. In this section I obtained, practically, a foreshortened 
view of the oblong cellular columns of Plate VIII. figs. 47 & 49, which reduced each 
column to the condition which one of the cells of Plate IX. fig. 55 would exhibit if 
elongated somewhat radially. The two radial and tangential sections now appear to 
harmonize without difficulty, and, unless I am seriously mistaken, they combine to 
sustain the genetic hypothesis which I have ventured to advance. This hypothesis is 
not without interest when viewed in connexion with various similar phenomena described 
in my previous memoirs ; all these examples exhibit the innermost cellular bark approxi- 
mating to the conditions which it ultimately attained in the cambium of exogenous stems. 
In the examples of the root which I have hitherto described, the maximum diameter 
of the vascular axis is not more than *075. But I possess others of larger dimensions, 
especially one for which I am indebted to Mr. Nield, of Oldham, in which the ligneous 
axis, deprived of its bark, measures fully an inch in diameter. All these specimens 
more or less resemble Plate IX. fig. 56, which represents a transverse section of a vas- 
cular axis, enlarged nine diameters. They exhibit numerous very strongly defined 
lines of circumferential growth, which are always irregularly concentric, each layer 
of vascular tissue being of unequal thickness on opposite sides of the stem, even 
when it forms a complete circle ; but they very frequently only form crescentic seg- 
ments of circles. We have already seen that the second exogenous layer of Plate VII. 
fig. 45 is of this character ; and several of the thin outermost ones at the lower part of 
Plate IX. fig. 56 exhibit the same aspect. Had the deficiency in these growths 
always been on the same side of the axis, we might have supposed that it was due to 
some peculiarity in the position of the growing plant relatively to light or moisture ; 
but this is not the case. The strongly marked boundaries of these concentric layers 
render this one of the most remarkable examples of exogenous growth in a Cryptogamic 
plant that has hitherto come under my notice. 
I have as yet brought forward no evidence to prove the correlation of these roots 
with the stems of Asterojphyllites ; but such evidence is not lacking. At an early stage 
of my inquiries I observed in several of my sections of these roots indications that some 
peculiarity existed in what ought to have been their medullary centres. In some of these 
there was only a triangular cleft ; in others I observed some clusters of vessels, of larger 
