226 
Journal of Agricultural Research 
Vol. XIV, No. 6 
Fig. t.—Solanum tuberosum: Diagram showing the course erf 
the vascular bundles in the stem and the mode of origin of 
the vascular supply of the leaves (longitudinal). (See p. 227 .) 
The vascular cylinder is not 
of uniform width throughout; 
projections into the pith occur 
in places (PI. 41, B). Its 
bundles are in part free, in 
part united by interfascicular 
cambium. The three comers 
of the stem are occupied each 
by a large bundle, and be¬ 
tween each two of these a 
smaller bundle is found (PI. 
27, A). The vascular tissue 
follows naturally a longitudi¬ 
nal course in the stem, and 
its arrangement is closely 
related to the scheme of phyl- 
lotaxis. In traversing the 
stem in a vertical direction 
these bundles always remain 
approximately at the same 
distance from the center, en¬ 
tering the leaf as a whole or 
in part. Those bundles or 
strands of vascular tissue 
which pass out into a leaf are 
known as leaf-trace bundles. 
The relation of these to the 
stem is of much importance, 
since each leaf trace not only 
supplies the leaf to which it 
belongs with water and min¬ 
erals, but also translocates 
the plastic materials manu¬ 
factured in the leaf. The 
diagram (text figure 1) and 
the figures of Plates 27 and 28 
show the course and origin 
of the various traces and 
their relation to each other 
and to the stem. A study of 
the petiole of the leaf shows 
that the vascular tissue, 
which occupies a semicircular 
area, consists of five groups: 
Three large groups and two 
smaller ones, the latter lying 
