730 PROFESSOR W. C. WILLIAMSON AND DR. D. H. SCOTT ON THE 
is doubtful, however, whether the two kinds of elements really had any special 
relation to one another, though, on the other hand, it is possible that some of the 
cells at the edges of the sclerotic bands may themselves have been of a secretory 
nature.* 
The outer cortex has the familiar “ Dictyoxylon” structure, consisting of alternate 
radial strands of sclerenchyma and parenchyma. On its outer surface are seated the 
curious emergences to which the specific name of “ Rachiopteris aspera” owed its 
origin. Asa rule, they are quite similar to those borne on the stem, but sometimes 
the outer sclerenchymatous layer is less developed and may appear merely as a 
thick-walled epidermis (see Memoir XVII., in which fig. 6 should be compared with 
fig. 7). The form of these protuberances is somewhat variable; usually they are 
bottle-shaped. Count Sotms-Lauspacat speaks of them as winged ribs, an ex- 
pression which might convey the idea that they are longitudinal ridges, of which 
the figures cited show the cross sections. This, however, is not the case. The 
petioles, it is true, are often somewhat winged on their upper edges, but such wings 
are quite distinct from the emergences, which may arise from any part of the 
surface. That they are not wings is shown by the fact that in a series of sections 
of the same petiole, their position never remains constant. Further evidence as to 
their real form is afforded by the fact that in longitudinal sections their appearance 
is essentially the same as in transverse section (see Memoir XVIL., fig. 8), and also 
by the fact that the cross section of the emergence itself is circular (loc. cit.). 
We have figured a peculiar form of these emergences, which is not unfrequently 
met with (see Plate 23, fig. 12). The emergence has the usual thick-walled superficial 
layer ; its internal parenchyma is generally thin-walled, but near the free extremity, 
there is a sharply defined group of well-preserved cells, with strongly marked walls 
and brown contents. This group appears to be limited externally by a somewhat 
flattened layer of cells. The structure certainly suggests a gland of some kind. In 
several cases, even when there was no such well-defined mass of specialized cells, 
the appearance of the parenchyma in the emergence is that of a tissue, which had 
been rich in cell contents. It is quite possible that a glandular function may have 
been discharged in certain cases by these emergences. 
4, Structure of the Lamina. 
Hitherto nothing has been known as to the histological structure of the leaflets of 
Lyginodendron. Now, however, we have several sections which show the tissues in 
great perfection. Two such sections are represented in Plate 19, photograph 7, and 
Plate 24, fig. 16. Both are from the series C.N. 1191-1198 so often referred to, and 
form part of leaves, the petioles of which show the characteristic structure of 
* For the sclerotic bands, see “ Organization,” Part VIL. Plate 52, figs. 11 and 12. 
+ “Fossil Botany,” p. 361. 
