496 Tansley and Chick. — On the Struchire 
The stele has a rounded or oval outline, a shape which 
is frequently distorted by the departure of the leaf-traces. 
Apparently the leaves normally have a § divergence, but the 
vertical distance between the exits of successive traces is very 
variable, so that some transverse sections show two in process 
of leaving the stele, while others pass across the exit of only 
one, and others again show the stele entirely undisturbed in 
this way. In a few cases the insertion of leaves and leaf- 
traces appears to be quite irregular. The endodermis is 
composed of cells which have usually less than half the 
diameter of the cortical ones, with an average of about 30-40 
on the circumference. The cells are filled with dense muci- 
lage. Their outer walls, separating them from the cortex, 
are thick and brown ; the radial walls are suberized in the 
usual way. 
The cells of the pericycle, which has but a single layer, are 
of about the same size as, and correspond accurately with 
their sister-cells of the endodermis. Occasionally a pericycle- 
cell is divided by a tangential wall. 
Immediately inside the pericycle comes the phloem, con- 
sisting of a one- or two-layered, almost complete zone of very 
small sieve-tubes, with practically no phloem-parenchyma. 
The number of sieve-tubes abutting on the pericycle is two 
or three times as great as that of the pericyclic cells. The 
breadth of the sieve-tubes is frequently twice as great in 
the tangential as it is in the radial direction. The tracheids 
of the xylem are often in immediate contact with the sieve- 
tubes of the phloem, i. e. there is no intervening layer of 
parenchyma such as Mr. Boodle has described in S. digitata , 
a layer almost universally present in Ferns. Occasionally, 
however, a few scattered parenchymatous cells are present 
between the xylem and phloem (a good many in Fig. 5), and 
in some of the larger steles this layer is constant and con- 
tinuous. Sometimes the layer of sieve-tubes is itself inter- 
rupted, so that in places the tracheids abut directly on the 
pericycle (Figs. 3 and 4). The tracheid-ring is one to two — 
very rarely more than two (Fig. 5) — cells thick, and, like the 
