172 Sargant and Arber .— The Comparative Morphology of 
At this period—that is, up to the time when the foliage leaves burst through 
the coleoptile—the seedling may be considered as an expanded embryo. 
Its ascending axis is the mesocotyl, crowned by coleoptile and stem-bud. 
The scutellum is inserted at the base of the mesocotyl, just where it passes 
into the primary root or descending axis. From the zone of this insertion 
spring several cauline roots (Text-figs. 9, 10, 11). 
In a seedling of this age the plumule must depend on the primary root- 
system until it has developed its own roots. The first two leaves draw their 
water at first from the lower roots through the mesocotyl into which their 
traces are prolonged. Similarly, they draw their proteid material from the 
endosperm through the scutellum. At this period there is a great demand 
for proteids in the neighbourhood of the first node, for new members are 
constantly being formed there—the roots of the nodal system, rudimentary 
leaves at the growing point, and buds in the axils of the leaves. Hence no 
doubt the necessity for connecting the plumular traces directly with the 
vascular system of the scutellum. Later on the green leaves will furnish 
a supply of proteids to this region by their own activity. 
The scutellum contains a single massive bundle, which follows the 
dorsal ridge and is equidistant from either surface. It is circular or oval 
in transverse section near the base, but towards the apex it spreads out into 
a crescent which repeats the convex outline of the dorsal epithelium. 
Occasional short branchlets are given off from the lateral wings, but they 
are not numerous, and they terminate at some distance from the epithelium. 
The carrying system is very well developed in this region of the scutellum, 
where the epithelium fringes both surfaces. On the whole, however, the 
outlying tissue is reached rather by lateral extension of the main bundle 
than by branching. 
The lignified elements of the extended bundle are scattered. They 
are most numerous in the median zone, where they often appear as a band 
of lignified tissue with protoxylem elements at either extremity. The 
bulk of the phloem fringes this band on the dorsal or convex edge. But 
groups of elements with thick contents, which probably represent phloem 
too, are found among scattered lignified elements on the ventral side of the 
xylem band. Lower down, where the bundle is oval or circular in outline, 
the xylem elements are collected on the ventral side of it, the phloem on 
the dorsal. 
At the apparent insertion of the scutellum, its bundle turns sharply 
upwards within the axis, but does not enter the stele (Text-fig. 6). Trans¬ 
verse sections through the mesocotyl show the slender stele in the centre, 
and on one side of it a massive inverted bundle—the scutellum trace 
(Text-fig. 16, p. 174, and PI. IX, P"ig. 5). They run upwards side by side to 
the real insertion of the scutellum at the first node. 
Throughout its upward course in the axis, the scutellum trace is clearly 
