198 Sargant and Arber .— The Comparative Morphology of 
of the coleoptile traces laid down. The latter behave, on the whole, very 
much as in Sorghum. They appear to give rise to px v px 2 ,px r 2 \ to the two 
xylem groups xy, xf ; and to the phloem groups ph v ph 2 , ph f v ph\. But 
minor plumular traces are inserted on them at critical moments, and further 
complications are introduced by the formation of cauline roots in the 
neighbourhood; hence the whole process is obscure, and would hardly be 
explicable but for comparison with Sorghum . 
No massive xylem bridge is formed by the union of coleoptile traces. 
A few elements of protoxylem are pushed forward from either side, and 
meet in the centre. There they are joined by similar elements from the 
midrib trace M. The mass of xylem elements in both traces stop far short 
of the centre, and form the two groups xy, xy' with their internal protoxylem. 
The central group remains undivided for a very short distance, then it splits 
up into px Y and/^g (Text-fig. 24, p. 197). 
Except for the temporary union of its protoxylem with px v the midrib 
trace M preserves its independence. A number of minor plumular traces 
are inserted on it at the nodes. In Sorghum , as described on p. 190, this 
trace is the only purely plumular element in the mesocotyl. In Zea , 
on the contrary, there are numerous plumular traces on either side of 
it. Coix is intermediate in this respect. One or two plumular traces appear 
to be prolonged into the mesocotyl on either side of M. About half the 
stele is derived from plumular traces ; the other half from cotyledonary 
(Text-fig. 24, p. 197). The ground-plan of the mesocotylar stele is essentially 
the same in all three genera, but Coix exhibits one feature—the separa¬ 
tion of px g from px\ —which, in our opinion, is primitive. 
Cauline roots are given off from the insertion zone as well as at the 
first node, and also at intervals along the mesocotyl. We were not surprised 
to find the elements characteristic of root xylem very well developed 
throughout this region. It is arranged with some regularity, as shown 
in Text-fig. 24, forming bays and crescents internal to the phloem. 
Euclilaena mexicana , Schrad. We possess but one series of sections 
from this species. It passes through scutellum and base of mesocotyl 
downwards to insertion zone and primary root. The mesocotylar stele 
resembles that of Coix ; but the insertion of the scutellum trace on it is 
much clearer, chiefly owing to the absence of cauline roots in this region. 
The two phloem groups in the scutellum bundle are quite distinct, and they 
enter the stele of the mesocotyl on either side of the ventral xylem. As 
these phloem strands pass up the stele they remain distinct from adjacent 
strands, and can therefore be identified with certainty as belonging to the 
scutellum trace. 
