428 Arber. — On the Roots of Medullosa anglica. 
The Phelloderm and Pericycle. 
A portion of a very thin transverse section is figured on 
PI. XX, Fig. 2. It shows the thin-walled tissue opposite 
one of the protoxylem-groups (p.x). The cells are fairly large, 
polygonal, or somewhat rounded parenchymatous elements, 
and are not very dissimilar in size or arrangement. 
Towards the periphery of the section, in the lower part of 
the photograph, a small portion of the periderm (p.d.) can 
be seen. Immediately internal to the phellogen ( ph.g) } some 
fairly large cells are shown lying on the same radii as the 
periderm-elements. This is no doubt phelloderm ( ph.d .), 
produced by the activity of the phellogenetic meristem. In 
certain sections in which the preservation is particularly 
favourable, the origin of these cells can be clearly traced 
from divisions of the phellogen layer. The thickness of 
phelloderm is probably quite small. At a short distance from 
the phellogen the radial arrangement of the cells is lost. 
Internal to the phelloderm, there occurs a fairly broad band 
of parenchymatous tissue ( p.c .), the cells of which are irregu- 
larly arranged. These elements constitute the pericycle. 
Conspicuous among the elements of the pericycle, single 
cells or groups of cells occur, with dark contents. These are 
regarded as ‘ secretory sacs V They appear to be ordinary 
parenchymatous cells in all the sections of these roots which 
I have examined. They therefore differ in structure from 
the gum-canals, which are so abundant in the petiole of 
Medullosa 2 . The ‘ secretory sacs ’ occur not only in the peri- 
cycle, but among the parenchymatous elements in all parts 
of the root, including those of the phloem and the primary 
xylem, as is clearly seen in Figs. 1 and 6. The dark colour 
of the cells is no doubt due to some organic change which 
the cell-contents have undergone before, or at the time of, 
preservation. The distribution of these ‘ secretory sacs ’ 
1 The term is used in the same sense as in De Bary’s Comparative Anatomy 
(Eng. edit.), 1884, p. 136. 
2 Scott, loc. cit., p. 99. 
