502 Tansley and Lulham . — A Study of the 
a constant strand in the centre of the second cylinder. After a time 
an endodermal rod appears in the central phloem and dies out again ; then 
a second, which likewise disappears, and so on. The stele is now much 
greater in diameter, and the space between its dorsal side and the internal 
cylinder considerably more. At the node the latter therefore sends up 
a distinct obliquely or vertically running column of tissue, which now 
invariably closes the gap and contributes a strand of tracheids to each 
of the free edges of the leaf-trace, which is horseshoe-shaped in section 
with an incurved hook of xylem on each side, or with actual incurved 
backwardly directed limbs. In the last case the tracheids running up from 
the inner cylinder supply the whole of these limbs. 
The adult type of trace, with its free edges again incurved and running 
forward, is now reached. At first this structure is only found at a short 
distance above the actual point of insertion of the trace and does not reach 
down to its base. In more advanced nodes, however, the forward curls do 
reach to the base and the ascending column of tissue from the second cylinder 
branches on each side into an inner and an outer limb, which supply the 
backward and the forward curl respectively. In the largest and most 
advanced nodes the diameter of the second cylinder has increased very con- 
siderably and the connexion with the outer cylinder and the base of the 
leaf-trace is made, not by the sending up of a column of tissue, but by 
the gradual raising and flattening of the roof of the second cylinder, till, 
as a broad almost horizontally running plate, its sides join the bases of 
the backwardly directed limbs of the trace. These last are often ex- 
tended forward, so that the junction is effected at a point considerably in 
front of the last section in the transverse series showing the departure 
of the front of the trace itself, a state of things which carries us back to the 
condition in which the simple internal xylem-strand only joined the external 
xylem after the departure of the trace. 
The forward curls of the trace are, in the largest forms, continued 
forward as flanges attached to the edges of the gap in the second 
cylinder. 
The third cylinder, which may vary, according to the diameter of 
the whole vascular system, from a solid protostelic condition through the 
Lindsay a- type to a solenostelic condition, probably first appeared as a local 
internal thickening of the gap in the second cylinder. With the exception 
of one case, we have always found it attached to the second cylinder at or 
near the point of closure of this gap, which in the vascular systems of the 
largest rhizomes is, on an average, about 8 mm. in front of the anterior 
roots of the leaf-trace, while in the absence of the third cylinder the gap 
in the second closes at an average distance of i-6 mm. in front of these 
anterior roots. 
