the Embryo and Seedling in the Gramineae. l 6 g 
The effect of these two structural alterations is that the stalk bundles 
are represented in the mesocotyl by a trace, distinct from the stele and 
parallel with it. The origin of this trace in Avena is suggested by its double 
structure and inverted orientation. 
(3) In the sheath the upward and downward segments of each lateral 
bundle must be united to the very base. (4) The downward segments on 
entering the stele must fuse with each other to form trace x (IV in Text- 
fig. 16, p. 174). 
These four modifications are sufficient to transform the imaginary 
type X into a vascular skeleton, with all the features which we consider as 
essential to the Avena type. Three more would be necessary to complete 
the resemblance to the actual Avena seedling. The sucker of X must 
become the scutellum of Avena ; the sheath-bundles must be capped with 
xylem elements ; and a xylem arch must be thrown from the scutellum 
trace to the mesocotylar stele. 
All these characters are in our opinion adaptive, and therefore of 
secondary importance for our present purpose. The greater differentiation 
of the scutellum, as compared with the sucker, is closely related to the rapid 
growth of the Grass seedling in comparison with that of other Mono¬ 
cotyledons. The xylem caps to the coleoptile bundles are doubtless organs 
for the excretion of water. We have already referred to the probable 
function of the xylem arch as a water-carrier. 
Our interpretation of the doubtful members of the Avena seedling is 
thus as follows : 
The scutellum is the sucking apex of the cotyledon. 
The coleoptile is the cotyledonary sheath—perhaps equivalent to a pair 
of stipules. 
The mesocotyl is the fusion of the cotyledonary stalk with the 
hypocotyl. 
The coleorhiza and the epiblast we consider as outgrowths from 
cotyledon or axis, or both, and of little morphological importance. 
The reconstruction of the missing link X , through which the Grass 
embryo and seedling are connected with those of hypogeal Monocotyledons 
in general, is intended to demonstrate how the following exceptional features 
of the Avena seedling may have arisen : the double scutellum trace and its 
inversion ; its indirect connexion with the bundles of the stele through the 
coleoptile traces ; the double .structure of the coleoptile bundles. Later on, 
the^ reconstruction of X will be shown to throw light on the Zea type also, 
with its more specialized mesocotyl. 
