286 
PROFESSOR W. C. WILLIAMSON ON THE ORGANIZATION 
peculiar condensations, g', of the cellular tissue of the leaf. In fig. 2, g", we find a 
similar condensation of the peripheral part of the leaf through which the bundle g' 
originally passed. 
Whilst the specimens described are Lepidodendroid branches identical with the 
type a of M. Renault’s three Lepidodendroid groups, they soon acquire an exogenous 
vascular zone, fie., assume a Sigillarian organization. I am indebted to Mr. Cash, 
Mr. Spencer, and Mr. Binns, of Halifax, and to Mr. J. Aitken, of Urmston, near 
Manchester, for numerous specimens illustrating this portion of the subject. 
Fig. 3 is the vasculo-medullary axis of a specimen like figs. 1 and 2, but with the 
beginnings of an exogenous zone, h. This exogenous growth commenced at a single 
point at the circumference of the vasculo-medullary axis, from which point it extended 
laterally in opposite directions, as well as radially, until it enclosed the vasculo- 
medullary axis in a complete cylinder, composed of vascular wedges separated by 
numerous medullary rays. In the specimen fig. 3 this growth has begun opposite to 
the point x, and has extended round little more than one-thircl of the circumference of 
the medullary axis. Whilst the transverse sections of the vertical portions of the 
foliar bundles are grouped closely round the axis at c, they are separated from it at c 
by the interposition of the exogenous zone, h. Their normal position is in the innermost 
cortical layer, which is necessarily pushed outwards by the growing exogenous zone. 
Fig. 4 is a segment of another section, enlarged 12 diameters, in which a further 
exogenous development has taken place ; and fig. 5 represents the vasculo-medullary 
axis and inner bark of another section from the same specimen, enlarged 28 diameters. 
In these specimens the exogenous zone has enclosed fully two-thirds of the circum¬ 
ference of the vasculo-medullary axis. As in fig. 3, the characteristic barred cells 
of the medulla are conspicuously numerous. The details of the structure of the bark 
seen in fig. 1 reappear in fig. 4, with the one exception, that the prosenchymatous or bast 
layer, f } instead of filling small detached crescentic areas at the base of each leaf, has 
now become a thick and continuous zone. The regular continuity of its peripheral 
border, f, makes it clear that the additions to its thickness have been made to that 
periphery, and not to its inner margin where it becomes merged in the outer paren¬ 
chyma, e. In more than one of my previous memoirs I have expressed my conviction 
that this has been the case, and the examples now described appear to confirm those 
conclusions. It further appears clear that this increase in the thickness of this bast 
layer is, like the development of the exogenous zone, an indication of a corresponding 
advance in the age of the stem. The leaf g, with its single vascular bundle g', exhibits 
a similar appearance to what we see in fig. 1, g'. 
Fig. 6 represents the central axis of another stem, in which the exogenous zone has 
completely encircled the primary vasculo-medullary axis. A considerable portion of 
tire latter structure has been destroyed, I presume before mineralisation. With these 
two exceptions, this section is a repetition of fig. 5. The innermost bark, b, is now 
more or less detached from the primary vasculo-medullary axis, and encloses the 
