168 de Fraine ,— The Seedling Structure of certain Cactaceae . 
may possess cotyledons in which one shows signs of reduction, or even of 
complete abortion, and they cite as instances of this Citrus Aurantium , in 
which the cotyledons differ in size and are not necessarily placed opposite 
to one another ; Pachira aquatica , which shows similar features; species 
of Abronia , in which one of the cotyledons is quite rudimentary; and 
Chacrophyllum and Corydalis, where only one cotyledon is present. They 
infer that ‘ there is some close connection between the reduced size of 
one or both cotyledons and the formation by the enlargement of the 
hypocotyl or of the radicle of a so-called bulb . . . and that one or both 
cotyledons, from being superfluous, decreased in size A further illustration 
of this reduction in size of the cotyledons is afforded by the Cactaceae 
seedlings. 
Additional evidence of this suppression is given by Goebel 1 for Trapa ) 
in which not only is there a difference in the size of the cotyledons, but the 
larger one arises as a terminal structure upon the embryo, while the smaller 
is lateral to the stem-bud. 
Finally, the experimental work of A. W. Hill 2 on the genus Cyclamen 
has shown that normally only one cotyledon is developed, which first serves 
as an absorbing organ, and, later, as an assimilating one ; the rudiment of 
the second cotyledon is always present, and should anything happen to the 
first cotyledon this rudiment develops into a normal green leaf. Further, 
in some species the second cotyledon may closely resemble the first one, 
while in others it is more like a foliage leaf. 
Thus there is no lack of evidence to show that the partial or complete 
suppression of one of the seed-leaves may occur in the Dicotyledons when 
the necessity for economy occurs * and it is possible that a similar 
suppression, owing to the adoption of a geophilous habit, may have taken 
place in the ancestors of Monocotyledons. 
For these reasons it does not appear necessary to postulate a 
theory of fusion to account for the occurrence of the monocotyledonous 
condition. 
The second theory relating to the evolution of the Monocotyledons 
has been put forward by A. W. Hill, 3 as the result of his observations on 
the germination of apparently monocotyledonous species of Peperomia , in 
which one cotyledon is hypogeal, and acts as a sucker, while the other has 
assumed the appearance and functions of a foliage leaf. He suggests that 
evolution along similar lines may have produced the normal seedling habit 
of such monocotyledonous orders as Araceae, and that the cotyledon and 
1 Goebel, K.: Organography of Plants, Part II, Oxford, 1905, p. 257. 
Hill, A. W. : The Seedlings of certain Pseudo-monocotyledons. Section K, Brit. Ass., York, 
1906. 
Hill, A. \Y. : Morphology and Seedling Structure of the Geophilous species of Peperomia, 
together with some Y iews on the Origin of Monocotyledons. Ann. Bot., xx, 1906, p. 395. 
