718 PROFESSOR W. C. WILLIAMSON AND DR. D. H. SCOTT ON THE 
also Plate 25, fig. 19, in which the same organs are shown in the cortex of a root). 
These organs are of universal occurrence, throughout the soft tissues of Lyginodendron, 
and may even be found within the limits of the vascular bundles. In a previous 
memoir* these structures have been described and figured as gum-canals ; on careful 
examination, however, we have failed to find any good evidence of an epithelium, 
while in satisfactory longitudinal sections, transverse or oblique walls, separating the 
constituent. cells, can generally be detected (see fig. 19). We therefore prefer to 
describe all these organs as secretory sacs. It is useless to speculate on the question 
what they secreted. 
Certain regions of the pith evidently retained the power of meristematic activity 
for some time, as is proved by the anomalous tissues to which they occasionally gave 
rise (see p. 722). Through the trace-gaps the pith is perfectly continuous with the 
pericycle. Where a bundle leaves the pith on its outward course, it is constantly 
accompanied by some of the sclerotic nests (see figs. 1 and 9); similar nests, however, 
are also frequent in the pericycle without special relation to the bundles. 
The pericyclic tissue consists, like the pith, of rather delicate, short-celled paren- 
chyma, together with sclerotic nests and numerous secretory sacs (see photograph 1 
and figs. 1, 5,6, 7, and 9). ‘he pericycle is often of great thickness, especially at 
the point where a leaf-trace is approaching its place of exit into a leaf (see photo- 
graph 4 and fig. 1). 
In all except the youngest stems the pericycle is bounded externally by a zone of 
radially arranged cells which we interpret as perzderm (see photograph 1, and figs. 1, 
5,6, 7, and 9, pd.). This layer, which was evidently formed from a phellogen, appears 
not to have originated until secondary growth had made some progress. In a stem 
with secondary wood about twelve elements thick the first tangential divisions had 
taken place in the outer layer of the pericycle.t The outer elements of the radially 
arranged zone have somewhat thick walls, while its inner cells are quite delicate, and 
no doubt constituted the cork-cambium or phellogen (see figs. 5,6, and 7). It is 
natural to suppose that this periderm must have ultimately caused the throwing off 
of the whole cortex, but we have no direct proof that this took place. The periderm 
curves outwards opposite the leaf traces, following their outer surface. In the speci- 
men shown in photograph 4, the periderm extends along the external surface of the 
outgoing trace as far as the base of the petiole. The length of the peridermal cells, 
as shown in longitudinal sections, is about equal to their tangential diameter. 
6. The Cortex. 
The cortex, as distinguished from the pericycle, may be divided into two layers. 
The inner layer is parenchymatous, while the outer consists of alternating radial 
* Wituiamson, “ Organization,” Part XVIL., p. 90, Plate 12, fig. 4. 
+ C.N. 1915 H. 
