874 PROFESSOR W. C. WILLIAMSON AND DR. D. H. SCOTT ON THE 
from cambium to cortex. Similar groups can be seen at several points of the section, 
and the preservation is sufficiently good to show that the phloém was formed in 
greater abundance opposite the primary bundles than in the interfascicular regions. 
The regular brick-shaped cells seen in fig. 14, are either cambium, or at least deri- 
vatives of the cambium, which have retained their original form. This specimen is 
one which already has a broad zone of secondary wood, about twenty-five cells in 
radial thickness. 
The external limit of the vascular cylinder is always quite sharp. This is due, 
however, to the almost constant presence of disorganized tissue on the boundary-line, 
not to the differentiation of any evident limiting pericycle. As to the presence of 
such a layer we have very little direct evidence. In the rare cases where the phloém 
is preserved, we sometimes find one or more layers of larger, thin-walled cells at its 
periphery (see fig. 13); these may have belonged to the pericycle. 
As regards the structure of the primary cortex, we find considerable variations 
among the comparatively few corticated specimens which are preserved. In the 
smaller twigs there is but little differentiation. The whole thickness of the cortex is 
made up of parenchyma, in which a few elements with thicker walls are scattered. 
(See Plate 72, photograph 1; Plate 77, figs. 1 and 2). Some of the cells have especially 
abundant carbonaceous contents, and may possibly represent secretory sacs. 
In other branches, of somewhat greater diameter, a more differentiated cortex is 
present, consisting of an inner and an outer zone, of distinct structure. (See Plate 72, 
photographs 2 and 8; Plate 77, fig. 83; Plate 78, fig. 12.) In ali these cases the inner 
zone is characterized by larger cells, with thinner walls, than those of the outer 
region. In the specimen illustrated in photograph 3, and fig. 12, in which the 
cortex is remarkably well preserved, the outer zone is itself differentiated, its cells 
becoming smaller and more sclerotic towards the periphery. The supposed secretory 
sacs may occur in any parenchymatous part of the cortex. We find, however, no 
indication of intercellular secretory canals. 
In a third type, already referred to, the outer cortex has sharply defined, wedge- 
shaped bands of sclerenchyma, alternating with thin-walled tissue. (See WILLIAMson, 
“ Organization,” Part XII., Plate 33, fig. 19, from C.N. 62.) 
The more complex cortex seems to be characteristic of the somewhat larger 
branches ; the number of corticated specimens is, however, too small to admit of any 
safe generalization. 
A definite endodermis at the inner margin of the cortex has not been detected 
with certainty. In several preparations however, the innermost cortical layer, where 
preserved, consists of regular, thin-walled cells, fairly distinct from the rest of the 
tissue. 
The above description sums up what we know of the primary structure of the 
internode, We have next to consider the modifications of structure presented by the 
