178 Sargant and Arber. — The Comparative Morphology of 
may enter the lateral root-plates from the plumular traces £ 2 and L\, 
though the bulk of their xylem—including all the protoxylem—unites 
with the ingoing branches of P and P' respectively. A more constant and 
important modification of the ground-plan is the formation of a massive 
xylem-bridge between the ingoing and outgoing branches of P and P' 
(Diagram III, Text-fig. 16, and PI. IX, Fig. 3). This bridge is very 
conspicuous in all the seedlings, and it must be slightly arched, for in 
serial sections followed downwards xylem elements commonly appear 
in the gap opposite M at a level higher than that in which the coleoptile 
traces begin to divide. As no formation of this kind is found in Zizania , 
where the division of the coleoptile traces between the stele and the scutel- 
lum trace is particularly clear, we are inclined to think that the formation 
of a xylem arch is not a primitive character. We have already (p. 167) 
suggested a possible function for it in Avena —to supply the scutellum with 
water from the lower roots by a shorter path than would otherwise be 
available. In the aquatic Zizania this adaptation is unnecessary. 
The stele of the mesocotyl is now complete (PL IX, Fig. 5). It is 
built up of the midrib trace M from the first leaf, of two plates (rp., rpf on 
which the nodal roots have been inserted, and of trace x t derived from the 
ingoing branches of the coleoptile traces. The root-plates when clear of 
the nodal roots lose their external xylem, but are always distinguished by 
a chain of large vessels. 
As soon as the nodal roots (r., r. in PI. IX, Fig.- 4) have differentiated 
their steles, they are in direct connexion through the root-plates with the 
main traces of the second and third leaves—already fully formed at that 
epoch. The first leaf is connected with the root-plates through a pair of its 
smaller lateral traces only (L 1 and L\), and perhaps by a few stray xylem 
elements from other traces as mentioned above. The main traces of the first 
leaf, together with a portion of each coleoptile trace, pass down the stele 
of the mesocotyl into the primary root. We have already shown how 
the xylem arch puts the scutellum also into direct communication with the 
primary root and the lower cauline roots. Thus the scutellum, coleoptile, 
and first leaf are dependent throughout their lives on the lower root-system, 
consisting of the primary root and three insertion roots (Text-fig. 12, p. 171). 
The second and third leaves are also connected with this lower root-system 
through the root-plates, for the latter bear the insertion roots below as well 
as the nodal roots above. Indeed, since the nodal roots do not become 
functional until the second and third leaves are unfolded, these leaves too 
draw water in their youth from the insertion roots. 
Little remains to be said concerning the lower root-system. Its relation 
to the mesocotyl has already been described. The primary root is heptarch 
or octarch ; the insertion roots hexarch or heptarch. 
