de Fraine .— The Seedling Structure of certain Cactaceae. 169 
so-called 4 first leaf ’ which are directly opposite to each other in the 
seedlings of some species of Arum and Arisaema may be the equivalent 
of the two cotyledons of Peperomia. He has further shown 1 that in 
Arisarum vulgare the 4 first leaf’ can perform the function of a cotyledon 
when the latter has become aborted or been torn off. 
Among the Monocotyledons the Araceae have been considered to be 
most nearly related to the Piperaceae. ‘ The affinities between two such 
simple orders as the Piperaceae and Araceae appear to be much more close 
and definite than between the anomalous Ranunculaceae and the highly 
specialized Liliaceae, and in the former case the modified pseudo-mono- 
cotyledonous Peperomias show definite homologies in their adult condition 
with the Monocotyledons.’ It seems probable that in some of the Mono¬ 
cotyledons, at any rate, the monocotyledonous condition has arisen from 
the dicotyledonous one along similar lines to those followed by the Pepero¬ 
mias in their development of the geophilous habit. 
In this connexion it may be noted that there has always been much 
difference of opinion as to the interpretation of the organs present in the 
grass embryo, and the view has been put forward that the scutellum and 
the germ-sheath represent highly differentiated parts of a single cotyledon ; 2 
in view of A. W. Hill’s work it is possible that they represent two cotyledons 
which have each taken on a separate function. A similar explanation may 
serve to explain the complicated structure of those monocotyledonous 
seedlings in which the seed-leaf is differentiated into an haustorium, a middle 
portion, and a sheath, e. g. Tradescantia , Cyperus , &c. In other mono¬ 
cotyledonous families, e. g. Juncagineae, Butomeae, and Alismaceae, the 
cotyledon becomes green, and according to Goebel 3 4 does not differ in form 
and structure from the first foliage leaves in any essential feature, though 
its anatomical differentiation is somewhat simpler. 
According to Miss Sargant, 4 however, the seedling anatomy of the 
Monocotyledons does not support A. W. Hill’s view, for the first leaf is 
characterized by a midrib and usually has lateral bundles in addition to it, 
while the cotyledon usually has no midrib; it has instead a double bundle 
or two single and quite distinct bundles, while in the cases in which 
a midrib is present it usually shows its double character during the 
transition. 
The presence or absence of a midrib seems then to be the chief 
difference between the cotyledon and the first leaf, and the question as to 
the possibility of the first leaf being a second cotyledon really rests upon 
the importance which can be attached to the 4 double ’ bundle. 
1 Hill, A. W.: The Origin of Monocotyledons. Ann. Bot., xxii, 1908. 
2 Rendell, A. B.: The Classification of Flowering Plants, i, p. 235. 
3 Goebel, K.: loc. cit., p. 408. 
4 Loc. cit., p. 178. 
