506 Tansley and Lulham. — A Study of the 
part of the outer stele (Text-Fig. 14, d-f). The internal cylinder, 
which has thus lost its dorsal part and become gutter-shaped, is now 
without a protoxylem, but it soon closes up and another protoxylem 
appears in its dorsal region. The behaviour at the next two nodes is 
apparently similar. 
In the smaller rhizomes showing the ‘ adult ’ type of leaf-trace (stages 
X and XI) the protoxylems are often less developed than in stage IX. 
In some cases, indeed, we have found it impossible to detect spiral 
protoxylems in connexion with the second (inner) cylinder at all. In 
other cases strands of spiral elements are sometimes confined to the 
leaf-trace itself, where they are to be detected on the insides of the 
two limbs of the lateral loops, but are apparently not continued, at 
least as spiral elements, into the inner cylinder of the rhizome. Here 
we may suppose that elongation, after the first formation of xylem- 
elements, is confined to the base of the petiole, and does not affect the 
rhizome itself. In other cases, again, these protoxylems are continued 
downwards and backwards into the dorsal part of the second cylinder, 
either joining to form a single strand, as in H, or remaining apart as 
two distinct strands, one on either side of the mid-dorsal line. A 
protoxylem-strand is sometimes found in the compensation-tongue of 
the second cylinder, which fills up the gap in the outer cylinder, but 
this, in the cases now under consideration, does not appear to be connected 
with the protoxylems which enter the leaf-trace. The protoxylems 
of the outer cylinder, on the other hand, are more numerous than in 
H, and a number of those occupying the dorsal side pass up into the 
arch of the leaf-trace. 
Stage XII represents the largest and most complicated type of 
vascular system, in which the connexion between the second cylinder 
and the outer one takes place far forward, often in front of the actual 
departure of the trace. Here the protoxylem-system of the dorsal region 
of the second cylinder and of the gutters which continue forward the 
lateral loops of the trace is much more developed, in correlation with 
the much more considerable development and longitudinal extension of 
these structures, which form the apparatus of connexion between the 
trace and the second cylinder. 
In the outer cylinder of this stage as many as eighteen protoxylems 
occur in a large rhizome. These are arranged closer together in the dorsal 
than in the ventral portion of the stele (Text-Fig. 15, A). A number of the 
dorsal protoxylems pass off with the main arch of the leaf-trace. The 
second cylinder may contain as many as nine protoxylems, though it often 
has only one or two, even in quite large forms. Of these there is always 
either one occupying the mid-dorsal line or a little to one side of it, or 
there are two, one on each side of the dorsal line. 
