49G 
PROFESSOR W. C. WILLIAMSON ON THE ORGANIZATION 
arrangement and size, the latter ranging between '005 and '0025. Numerous vascular 
bundles are seen passing outwards to the leaves, as at b. The more external portions 
of the bark have nearly all disappeared. The little that remains, f binding together 
the leaves, c, c , shows that it possessed the bast-layer, composed of vertically elongated 
prosenchymatous cells, with an outer investment of parenchyma—the usual structure 
of this portion of Lepidodendroid and Sigillarian stems. The leaves in this section 
exhibit nothing worth remarking except that the three c, c, c, have been intersected 
through their larger transverse diameters, whilst the two alternating ones, c, c, have 
been crossed close to the point where their bases spring from the bark. Their closely 
united rhomboidal bases have a diameter of about ^-g-ths of an inch. 
The next progressive step brings us to the specimen represented by fig. 5, which is 
a section of either a small stem or a very large branch found by Mr. Wunsch. It 
is represented three-fourths its natural size. We now find the central medulla, cl, 
about an inch in diameter, but its cellular tissues have disappeared through decay or 
mineralisation. At a we have the vascular medullary cylinder corresponding exactly, 
in all but size, with that seen in the specimen, fig. 3. But we now find externally to 
this cylinder a very thin layer, i, of barred vessels arranged in radiating lines. This 
zone is, of course, the “Cylindre ligneux” of Brongniart, and my exogenous zone, 
the existence of which, according to the French author, de facto, transfers the plant 
possessing it from the Lepidodendroid to the Sigillarian group—or, in other words, 
from the Cryptogamic to the Gymnospermous divisions of the vegetable world. Ate 
we again have the delicate middle parenchyma seen at fig. 3, e, but often disturbed 
by a stalagmitic arrangement of the carbonate of lime, as at e. This inner tissue is 
less perfectly preserved than in the specimen, fig. 3, but sufficiently so to establish 
the perfect identity of the two structures. We pass by a somewhat rapid transition 
from the delicate parenchyma of this middle bark to the much coarser parenchyma 
of what, in my previous memoir, “ I have spoken of as the middle bark, fig. 5, g ; 
and which yet more externally, fig. 5, A, passes gradually into the radiating lines 
of prosenchymatous cells of what may be designated the bast-layer. In these two 
outer portions the foliar bundles are numerous and distinct. Now if any unbiassed 
student compares the arrangement of these tissues, as shown in this fig. 5, with those 
seen in many of my previous figures of Lepidodendra and Halonice (e.g., Plate 24, 
fig. 1, and Plate 26, fig. 13, of memoir, Part II.), and then further compares these 
with Brongniart’s own figures of his typical Lepidodendron, as represented in Plate 
20, figs. 2, 3, and 4, of vol. 2 of his ‘ Vegetaux Fossiles,’ it will be seen at a glance 
how complete is the identity of these detailed organizations, and what violence is done 
in separating them widely asunder merely because nature met additional nutrient 
wants by an addition to the nutrient machinery. In fig. 5 of this memoir we see the 
beginnings of this process, which in the larger stems underwent a much further exten¬ 
sion. Fig. 6 represents a transverse section of one of the vascular axes already referred 
* See memoir, Part II., Plate 24, fig. 1, h. 
