206 Sargant and A rber.—The Comparative Morphology of 
ready splitting of the bundle-trunk to correspond with the twofold structure 
of the coleoptile might indeed suggest reversion to an originally dual 
symmetry. Otherwise the increase in mass of vascular tissue might perhaps 
have been effected by the division of the bundle-trunk into three or four ; 
or by the more vigorous branching of a single trunk. We cannot, however, 
attach much weight to this argument, considering our ignorance of the laws 
of adaptation^ 
Comparison of the Triticum type with the Avena type. 
The vascular symmetry of the Triticum seedling is easily derived from 
that of Avena by suppression of the mesocotyl. And this suppression 
is not merely hypothetical, for in certain Triticum seedlings we have found 
a very short mesocotyl, in which the anatomy is precisely comparable to 
that of Avena (ante, p. 200). We have not made any similar discovery in 
Hordeum vulgare, where the mesocotyl seems always absent. In other 
respects its vascular symmetry is that of Triticum, substituting two bundles 
for one in the scutellum. We have already discussed the significance of 
this character. 
II. The Anatomy of Certain Other Monocotyledonous 
Seedlings compared with tpiat of the Grasses. 
The three types of seedling structure distinguished by Van Tieghem 
in the Gramineae appear at first sight astonishingly different. The absence 
of a mesocotyl distinguishes the Triticum type from the others, and modifies 
its anatomy profoundly. The insertion of the scutellum trace on the stele 
occurs at the base of the hypocotyl in Zea, while it is postponed to the first 
node in Avena . On the other hand, certain anatomical features found in 
all the types make comparison with other Monocotyledons difficult. For 
example, the two opposite and equivalent bundles in the coleoptile dis¬ 
tinguish it from the scutellum, with its single bundle on the one hand, and 
from the plumular leaves on the other. It cannot be satisfactorily treated 
either as the cotyledon or as the first leaf. Again, the double structure of 
the coleoptile bundles—particularly plain in Avena, Sorghum, and Zea, and 
very clearly indicated in Triticum —can hardly depend on the double 
character of the scutellum trace, since only one-half of the latter reaches 
each coleoptile bundle. Finally, in all the types the coleoptile bundles 
appear to arise at the first node, partly from the scutellum trace, and partly 
from the stele of the mesocotyl. 
These features are explicable if all three types are derived from an 
imaginary ancestor X, with the seedling skeleton suggested in Text-fig. 7, 
p. 166. We have already considered the modifications of structure necessary to 
convert that ancestor into a seedling of the Avena type (pp. 168-9). None of 
