182 Sargant and Arber .— The Comparative Morphology of 
vascular tissue consequent on the aquatic habit, and partly to the complica¬ 
tions introduced by the number of roots found at the first node, and by the 
formation of a vascular girdle on which they are inserted. On the whole 
this is true, but in one important character the nodal structure is clearer in 
Zizania than in Arena , namely, in the insertion of the scutellum trace. It 
divides into two branches before reaching the stele. Each branch joins 
a coleoptile trace. No xylem arch is formed as in Arena , and there 
is therefore no direct connexion between the scutellum trace and the stele. 
One or two xylem elements may perhaps be found by diligent search to go 
from one to the other, but the rule is that the scutellum trace divides itself 
completely between the coleoptile bundles P and P'. The other elements 
of P and P' are prolonged downwards into the stele as the compound 
trace 
When the mesocotylar stele is fairly formed, it contains the trace x 
derived from coleoptile traces, and a trace opposite to it which probably 
represents M, the midrib of the first leaf (Diagrams III and IV, Text- 
Fig. 19, p. 180). There are also one or two xylem groups on either side which 
probably correspond to the lateral xylem plates in Arena. These lateral 
groups become insignificant as we descend the mesocotyl, and are very soon 
reduced to two—rarely three or four—lateral vessels of large lumen, placed 
symmetrically with regard to M and x near the periphery of the stele. 
The mesocotylar stele is very little lignified, except just below the first 
node, and again in the region of transition to a root-structure. Even at this 
lower level, very accurate staining is required to bring out the xylem 
elements at all in contrast to surrounding tissues. In two seedlings only 
among the five examined is it possible to make out the symmetry of the 
root-steles. In both cases that of the primary root is pentarch, with an 
inclination to become tetrarch by fusion of two xylem rays. The cauline 
roots are 6 to io arch. The method of transition is obscure in all the 
seedlings. 
Thus the vascular structure of Zizania may be described as that 
of a slender Arena , with imperfect lignification and no cauline roots spring¬ 
ing from the insertion level of the scutellum. The imperfect lignification 
is, no doubt, connected with its aquatic habit. Whether the absence of 
insertion roots is a consequence of this habit too can hardly be determined 
without observations on the growth of Zizania under natural conditions. 
The most important anatomical character from our point of view is the 
very clear way in which the scutellum trace is built up from portions 
of both coleoptile traces, and the absence of direct connexion between it and 
the stele of the mesocotyl. If the object of the xylem arch in Arena be to 
put the scutellum into direct communication with the water-supply from the 
lower root-system (ante, p. 167), then the absence of this character is readily 
