160 MR. P. GROOM—CONTRIBUTIONS TO THE 
mass of mucilage. Around this is a vacuole across which stretch 
a few threads of mucilage connecting the central mass with a 
much larger peripheral sponge-work of mucilage*. The nucleus 
hes crushed flat against the wall. 
The endocortex varies considerably in structure. Apart from 
the distinctions already mentioned, it differs from the mediocortex 
in that the raphide-mucilage cells are smaller in size and fewer 
in number. Atthe base of the root the whole of the endocortex 
is converted into cells having very thick lignified walls with 
numerous simple pits. These cells preserve their nuclei and 
protoplasmic contents, and may contain starch. This change 
even invades the mediocortex, so that one to two of its innermost 
layers have these thick lignified walls, but contain mycelial 
masses. Thusacomplete section of such a root would display :— 
(1) Piliferous layer. (2) 3-4 layers of exocortex. (3) Nine 
layers comprising the mediocortex, of which the two innermost 
have thick lignified walls. (4) Five layers constituting the 
lignified endocortex. (5) Endodermis and central cylinder. 
Tracing such a root towards the apex, the number of lignified 
layers diminishes, the outer ones first losing their thick walls, till 
there is only one layer immediately outside the endodermis which 
is lignified. This single layer is notably interrupted opposite the 
xylem-bundles by thin-walled cells, which obviously function as 
passage-cells and lie immediately outside the slightly protruding 
passage-cells of the endodermis (Pl. IV. fig. 5). This last layer 
loses its thick walls. The object of this large production of 
mechanical cells will be discussed later. 
The endodermis is a single layer consisting of cells with thin 
suberized walls (fig. 5). 
The pericambium (fig. 5) forms a single complete layer, and in 
some spots it is two cells thick. Each cell is brick-shaped and 
protoplasmic. 
There are numerous (20-28) alternating bundles of xylem and 
phloém. 
The xylem is made up of tracheides and parenchyma (PI. IV. 
fig. 5). The tracheides increase in calibre from without inwards, 
and have transverse-bordered pits, so that the thickening varies 
from pitted to scalariform and reticulate. Each xylem-bundle 
* It must be recollected that the observations were made on material fixed 
with ordinary methylated spirits, so it is impossible to say how far this 
resembles the original structure. 
