286 
PROFESSOR ~W. C. WILLIAMSON ON THE ORGANIZATION 
exhibiting a tendency to elongation in the surfaces of the leaves (Plate XLI. fig. 5, l). 
I have had the utmost difficulty in determining whether or not vascular bundles were 
prolonged into the leaves of these young Lepidodendroid branches ; I cannot find such 
in the smaller twigs, but I have detected them in two specimens rather larger than 
fig. 1 ; and in some others I trace vacant spaces in the leaves which, I doubt not, were 
occupied by similar bundles. In one transverse section, like fig. 1, I discover two small 
bundles at a little distance from the central cylinder. 
Various sections in my cabinet exhibit a gradual increase in the size of all the con- 
centric layers of tissue just described. Plate XLI. fig. 8 is a transverse section of one 
of the larger vascular cylinders, drawn to the same scale as figs. 3 & 4. The cylinder in 
this instance is nearly uncompressed, and has a diameter of -0625, whilst the area 
occupied by the cellular medulla has attained to a diameter of *03. The barred vessels 
composing the cylinder have also undergone a corresponding increase in their dimen- 
sions, the largest of them having attained to a maximum diameter of *005. The 
expansion of the cylinder is but partly due to the increase in the size of the vessels. 
There has been a simultaneous increase in their number. In the three figures 3, 4, & 8, 
Plate XLI., every vessel in the respective sections has been copied with geometric accu- 
racy, so that the drawings may be relied upon as correct transcripts of the sections. We 
find that in fig. 4 there are about eighty vessels in the entire cylinder ; in fig. 8 there 
are more than four times that number. It will also be observed that a large number 
of very small vessels is developed at the periphery of the cylinder, these being apparently 
the newest growths of the series. 
Plate XLV. figs. 31 & 32 represent the external aspect of the leaves at this stage of 
the plant’s growth. They are ovato-lanceolate, and very closely imbricated. The central 
longitudinal keel is more or less prominent, as is also shown to be the case in their 
transverse sections. I have found a few fragments in which this dorsal ridge is impressed 
with several transverse indentations, as represented in fig. 32 : whether this condition 
represents a distinct species or a mere variety I am unable to say ; at all events it is not 
the common form of these leaves. In their general habit these twigs closely resembled 
the Lycopodium Saururus figured by Bkongniart*. Small as these leaves are in this 
young state, they gradually develop into thick scale-like structures, which ultimately 
attain to considerable dimensions. 
The next step takes us to Plate XLII. fig. 9, where we find the plant assuming the 
form of the young branch of a JDiploxylon. The specimen represented is much com- 
pressed, so that the cellular medulla is obliterated, or nearly so. The two inner sides of 
the vascular medullary cylinder (c) are thus forced into close contact. The thickness of 
this cylinder, from its inner to its outer surface, has been about '044, that of Plate XLI. 
fig. 8 having been about '014; hence we see that this portion of the plant has here 
undergone a yet further increase in the number of its component vessels. But a new 
element now makes its appearance for the first time. The vascular medullary cylinder 
* Ye'getaux Fossiles, tome ii. pi. i. fig. 1 . 
