220 Sargant and Arber .— Comparative Morphology of 
and stalk of the cotyledon, on their way to join the stele of the hypocotyl. 
The Zingiberaceae may represent one of those forms, the Gramineae another ; 
and the comparatively simple sheath of Elettaria may still serve to indicate 
the manner in which the coleoptile of Avena was evolved through a distinct 
line of descent. 
The sheath of Commelina may represent an early stage in the evolution 
of the coleoptile, demonstrating how it might be derived from the simple 
sheathing base of the cotyledon, in the most usual form of epigeal germina¬ 
tion ( Allium , Anemarrkena , &c.). And in this genus, too, interest is at once 
aroused and baffled by the existence of apparently primitive characters. 
We have already remarked on the anatomy of the hypocotyl, which is that 
of a typical Dicotyledon. Solms-Laubach has shown that the stem apex 
in the embryo of Commelina is terminal like that of a Dicotyledon, while 
in the typical Monocotyledon it is lateral. 1 
In conclusion, we think that the key to the morphology of the Grass 
embryo lies in the morphology of its seedling, as interpreted by comparison 
with the seedlings of other Monocotyledons. This comparison is not easy, 
for the anatomy of the Grass seedling is complicated, and very distinct 
variants are found within the family. We have shown that all these variants 
can be derived from an imaginary type X (Text-fig. 7, p. 166). The scutellum 
then represents the sucker of the X cotyledon, and the coleoptile its sheath, 
and in both cases this is the most natural interpretation of their anatomy. 
The vascular skeleton of X is that of a hypogeal Monocotyledon, and is 
sufficiently near that of the Zingiberaceae to be derived from it without 
difficulty. But to derive the vascular skeleton of the Avena type from that 
of X requires one considerable assumption ; that the stalk which should 
connect scutellum with coleoptile has become united with the hypocotyl. 
We maintain that the mesocotyl is more likely to have arisen in this 
way than as the elongated node, which Van Tieghem suggested ( 72 ). The 
latter hypothesis does not explain the presence of the inverted trace found 
within the mesocotyl of certain forms by previous observers. 2 On our view 
this trace represents the stalk, and is the last vestige of its independence. 
1 Wettstein ( 1 . c., p. 811) remarks on the affinities of the Enantioblastae, including the 
Commelinaceae, with the Liliiflorae on the one hand and the Gramineae on the other. 
2 Miss Lewin (’ 87 , p. 22 and PL III, Fig. 46); Bruns (’ 92 , p. 23) ; Schlickum (’ 96 , p. 58 and 
PL V, Figs. 147, 157). 
