188 Sargant and Arher .— The Comparative Morphology of 
groups of soft bast which constitute a centre. The outline of the double 
group is kidney-shaped, but the phloem elements of the scutellum trace 
seem to run chiefly into the nearer of the two segments. When these have 
finally separated, the xylem crescents are also defined, and the mesocotylar 
stele has assumed its characteristic structure (PL X, Fig. io). 
Thus the scutellum trace supplies a single isolated element of proto- 
xylem (px 1 .) to the mesocotylar stele, and also contributes at least half of the 
phloem to each of the two lateral centres. It does not account for all the 
lateral phloem, nor for any part of the lateral xylem crescents, and it 
is quite unconnected with the median bundle. 
Below the scutellum insertion the vascular symmetry of the mesocotyl 
is sharply separated from that of the primary root by the formation 
of a plate of vascular tissue. In the upper part of this region the phloem 
of the median bundle can still be traced distinctly. Opposite to it is 
a single phloem group, representing the elbow of the scutellum trace as 
it turns on itself. The massive xylem crescents which define these two 
groups internally embrace and nearly meet round them. The centre 
of the stele is occupied by a plate of metaxylem uniting the crescents with 
each other. This formation recalls a similar structure in the transitional 
region of Fritillaria imperialist L. It persists through a few sections 
only. Below these the phloem and xylem become inextricably mingled, 
and finally settle down into normal root-structure. 
The primary root is polyarch, with nine or ten xylem rays. Just 
within the protoxylem of each ray is a single well-lignified element of wide 
lumen. When cut obliquely it is seen to be a vessel with pitted walls. 
The ray commonly ends with this vessel, but sometimes a few smaller 
lignified elements lie next it on the inner side. 
In the centre of the root-stele are many xylem elements scattered 
among unthickened cells which probably represent conjunctive tissue. 
These xylem elements are of different sizes; their walls more or less 
thickened, and more or less lignified. They do not form a continuous 
xylem plate, and are distinct from the rays. Among them are always two 
elements of very large lumen. Their walls are only slightly thickened ; 
but in the older seedlings examined they are completely lignified, and can 
be seen-—where cut obliquely—to be uniformly pitted. Each is bordered 
by a single row of small elements, mostly unlignified, and some of them thin- 
walled. The two large vessels can be followed upwards through the insertion 
node, and are found to be continuous with those which separate the median 
bundle of the mesocotyl from the lateral bundles (rx. in PI. X, Fig. io). 
Returning to the stele of the mesocotyl, its structure is slightly 
1 Sargant, E. (’ 03 ) : A Theory of the Origin of Monocotyledons, founded on the Structure of 
their Seedlings. Ann. of Bot., vol. xvii, p. 24, and PI. Ill, Fig. 4, 1903. 
