2o8 Sargant and Arber .— The Comparative Morphology of 
sented elsewhere. Thus the formation of the scutellum trace from the 
coleoptile bundles is better shown in Zizania aquatica , their double structure 
in Sorghum vulgare , and the coleoptile traces within the stele retain their 
identity longer in Coix Lacryma-Jobi. 
Nor is it strictly accurate to say that the Triticum type can be derived 
from the Arena type in one way, and the Zea type in another. Our 
hypothesis is better formulated thus : the seedling skeleton of the common 
ancestor from which are descended all the genera we have examined 
probably resembled the Arena type in the possession of a mesocotyl, and 
in the less complete union of the two members from which it is derived. 
A greater number of the structural variations depending on these characters 
are found in the Arena type than in the others, and it may be called 
primitive with regard to them, since it approaches more nearly to our 
conception of an ancestral form within the Gramineae. 
This ancestral form, which may be referred to as G , is not to be con¬ 
fused with type X figured in Text-fig. 7, p. 1 66 . X has none of the anatomical 
features characteristic of the Gramineae, but illustrates a skeleton from 
which such features might be readily derived on the one hand, while on the 
other it has the seedling characters of a hypogeal Monocotyledon. 
We shall now describe the seedling structure of certain Monocotyledons 
which approach more or less closely to the imaginary type X , in order to 
justify our use of that construction in explaining the anatomy of the Grass 
seedling. Before entering on this subject, however, we must explain the 
absence of any reference to the Cyperaceae. We naturally expected that 
the seedling structure of this family would throw light on that of the 
Grasses. But the vascular skeleton of the species we have examined is too 
much reduced to be significant. Even the seedlings of the comparatively 
robust Cyperus alternifolius , L., and C. natalensis , Hochst., are as small and 
tender as those of an aquatic plant. 
We have already stated that Van Tieghem considered the mesocotyl 
to represent an elongated node, whereas we look on it as a fusion of the 
hypocotyl with part of the cotyledon. On either hypothesis the coleoptile 
of the Grass seedling is identified with the upper or stipular sheath. 
An upper sheath is not universal even among hypogeal Monocotyle¬ 
dons, and cases in which it contains vascular tissue are comparatively rare 
outside the Glumiflorae. Such cases are, therefore, likely to be instructive, 
and we have already referred to some of them. We propose to discuss 
these and others in greater detail, beginning with the seedlings of Elettaria 
Cardamomum and its allies. 
