496 
Tans ley and. Lulham. — A Study of the 
of the complexity of structure of the third cylinder with the diameter 
of the whole vascular system : — 
Structure of Third Cylinder. 
Cases. 
Horizontal diameter of 
First Cylinder in mm. 
Third cylinder absent (adult type) 
5 
i*6 — 2.2 Average 1.9 
Third cylinder protostelic alternating with its 
absence 
2 
3 -o 
„ 3-0 
Third cylinder protostelic throughout 
*9 
1.9 - 4.0 
„ 3 *i 
Third cylinder solenostelic at nodes only (simpler 
in internodes) 
10 
3-5 - 5 * 1 
„ 4 - 1 
Third cylinder solenostelic throughout 
11 
w 
00 
1 
rji 
to 
„ 4.6 
Summary of the Progressive Complication in the Vascular 
System of Rhizome and Leaf-Trace. 
The following paragraphs summarize the preceding section. The 
successive complications in the structure of the vascular system are treated 
as forming a single series with the exceptions noted in the course of the 
summary; they are illustrated by the accompanying tables (pp. 498-503). 
The simplest structure available consists of a slender cylinder of 
xylem surrounded by pericycle and endodermis, with no characteristic 
phloem present. The first leaf-trace is a strand of similar structure. 
The stele of the rhizome now increases considerably in diameter and in 
number of elements, and a well-marked strand of phloem containing 
sieve-tubes and parenchyma appears in its midst. Immediately afterwards 
external phloem appears at several points on the periphery of the xylem, 
and is connected with the phloem of the first roots, which arise in this 
region. The second leaf-trace consists of a slender strand of xylem 
surrounded by a thin layer of phloem, which is contributed to by both 
the internal and the external phloem of the stele. The internal phloem 
now increases considerably in bulk, and there arise in it a few large 
isolated endodermal cells. The third leaf-trace is an arc of xylem covered 
with a layer of phloem, of which that on the abaxial and lateral faces 
is continuous with the external, and that on the adaxial concavity with 
the internal phloem of the stele, while the adaxial endodermis of the trace 
is continuous with the internal endodermal cells of the stele. An internal 
ridge of the xylem projects into the internal phloem of the stele on 
one side of the base of the third trace. The internal endodermal cells 
eventually join to form a continuous endodermal strand. At subsequent 
nodes the internal ridge of xylem again appears, and at length a node 
is reached in which the ridge is continued forward in front of the departure 
of the trace for a short distance, and then becomes free in the internal 
