390 Chandler .- — On the Arrangement of the Vascular Strands 
ment in the central part of xylem) become definitely arranged in a small 
excentric group, and well-marked sieve-tubes, two or three in number, 
appear among them. The fourth leaf-trace, quite simple and leaving but 
a small gap, is then given off exactly opposite the stolon. The gap is 
merely bridged by the endodermis. Just before the trace is differentiated, 
an isolated section presents an appearance very similar to that described 
by Tansley and Miss Lulham for Lindsaya *, the excentric position of the 
internal phloem being very striking. The explanation of this appearance 
is, of course, very simple, since the vascular tissue of the stolon, although in 
perfect union with that of the stem, lies alongside the latter for some 
distance as a kind of keel (Figs. 133, 134). Gradually, however, the union of 
the two strands becomes more perfect, and the cauline strand once more 
assumes its cylindrical condition. The leaf-gap is very quickly closed, 
and a root, with densely sclerosed cortical tissue, then joins the stem at 
a point opposite the leaf. 
As soon as the stem-strand has regained its normal cylindrical 
condition, sieve-tubes can no longer be distinguished in the pith, the 
cells of which resume their scattered arrangement among the central 
tracheides of the xylem. The somewhat indefinite external sieve-tubes 
show a tendency to arrange themselves in groups, a feature which is more 
or less constant in older plants. The next change is the somewhat sudden 
re-grouping of the parenchyma cells to form a central patch in the xylem, 
followed by the appearance of a few sieve-tubes. At the same time 
the pericycle increases in width, the cells of its inner layers not being 
in radial rows with the cells of the outer layer, which, as before noted, 
are in seriation with the endodermal cells. 
The vascular strand has meanwhile increased considerably in size, and 
the next leaf-trace, though still small, is much larger than its predecessors. 
It consists of an arc of xylem surrounded by phloem, and leaves a gap 
which is very quickly closed. The second stolon then joins the stem, 
its vascular tissue appearing as an immense root-trace. As before, its 
junction occurs exactly opposite a new leaf-strand, whose exit leaves a gap 
into which the ground-tissue is differentiated to a slight depth. A trans- 
verse section of the stem at this level shows a rounded mass of tracheides 
and xylem parenchyma with a shallow gap on one side containing sieve- 
tubes and parenchyma, which also form a continuous sheath round the 
xylem as a whole. The ground-tissue, dipping slightly into the gap, 
is very striking, since its strongly sclerosed polygonal cells contrast sharply 
with the very definite small-celled endodermis. The phloem in the leaf- 
gap occupies an approximately central position in the xylem below the 
level of the stolon, its excentricity, as before, being solely due to the 
augmentation of the cauline vascular tissue by the stolon bundle (Fig. 135). 
1 Loc. cit. 
