42 
PROFESSOR W. C. WILLIAMSON ON THE ORGANIZATION 
land by G. Grieve, Esq., were some stems having a similar structure, but of larger 
size than any I had met with in Lancashire ; but, though exquisitely preserved, the 
fragments were internodal ones, and not sufficiently long to exhibit the structure of 
the nodes. 
I shall adopt in this memoir the plan which I followed in the third of this series of 
monographs, in which I treated of the Burntisland Lepidodendroid plants ; I shall com- 
mence with the youngest twig of the Lancashire type which I have met with, and then 
trace the gradual development of the organism through the addition of successive 
exogenous growths by which the twig was thickened and finally converted into branch 
and stem. 
The young Twig . — Plate I. fig. 1 * represents a transverse section of a condition of 
which I have obtained several examples, and which evidently exhibits a twig or branch 
in its very young state ; c is a central vascular bundle which has a maximum diameter 
of -016 f between the most distant tips of the slender divergent angles of its triangular 
section, the entire section, including the bark, being about *06 in diameter. The bundle 
consists of a group of vessels, not arranged in any regular order ; the maximum diameter 
of the largest of them is not more than ‘0029, whilst that of the smallest is not more 
than -0007. The triangle, as seen in this section, is very slender, its arms being long in 
proportion to their breadth. The larger vessels occupy its central portion, the smallest 
being found at the extremities of each of the arms. 
A double inner bark (g) has disappeared from this specimen, though traces of it are seen 
in some similar but otherwise less perfect ones. It appears to be of the same nature as 
that which will be described in the more matured stems. This double inner layer has 
been surrounded by an outer bark (k), with a maximum thickness of about -016. It is 
composed of compactly aggregated cells, with very thick walls, and which are of about 
the same diameters as the central vessels. A few of these cells have a diameter of -0029 ; 
many others are yet smaller, whilst the great majority range between -0015 and *0007. 
A remarkable feature of this outer bark is its indentation, on each of its three sides, by 
a deep constriction ( Jc These constrictions correspond with deep grooves which 
channel the surface of each of the internodes, but which disappear as they approach 
the nodes J. The position of each groove is opposite each concave lateral surface of the 
central vascular bundle, when the latter has not been displaced from its normal 
* I may observe that, to facilitate the comparison, of these specimens, all the following figures have been 
drawn to the same scale, being enlarged 30 diameters, 1, 2, 3, 4, 5, 9, 10, 11, 12, 14, 15, 16, 18, 19, 20, 21, 
27, 34, 45, 50, 51. 
t These and all subsequent measurements are given in decimal parts of an inch. 
+ I find precisely similar grooves in a small plant of SpJienopJiy Hum Schlotheimi (Plate Y. fig. 26) in the 
museum of the Owens College. In this instance the grooves (Plate Y. fig. 26, Jc') are never vertical to any 
of the leaves of a verticil, but appear always to be intermediate between two of them. It also appears, from 
the same specimen, that the positions of these grooves do not change in alternate nodes, but that they form 
continuous lines down the stem, interrupted only at the nodes. Plate Y. fig. 26, 7c', is an enlarged view of 
part of the internode and lower node of Jc. 
