148 Sargant. — The Reconstruction of a Race of 
Dicotyledons. This hypothesis was readily accepted by observers already 
convinced on independent grounds that Monocotyledons were the elder 
race. Now that these grounds are given up, the embryological evidence 
stands alone and must be criticized on its own merits. 
The chief difficulty in the way of accepting this solution is that the 
cotyledon of Alisma , for example, is to all appearance a terminal structure. 
If this represents the ancestral form, the two cotyledons of Capsella must 
be considered as due to fission of a terminal member. But the typical leaf 
is formed laterally on the growing-point of an axis. The rest of the 
growing-point forms other lateral members, at the same time reproducing 
itself. Can a member formed from the whole of the growing-point — that is, 
terminally to an axis which then ceases to form new members — be a true 
leaf? 
Many morphologists think that it cannot. If we consider the pro- 
embryo to possess an axis, there is no denying that the single rudi- 
mentary cotyledon of Monocotyledons, or the bifid rudiment which re- 
presents the pair in Dicotyledons, is for a time apparently terminal. 
Hence some botanists maintain that the cotyledons cannot be considered 
as true leaves. 
Those who hold this opinion belong to two schools. The first draws 
a distinction between Monocotyledons and Dicotyledons. The cotyledons 
of the latter may be regarded as lateral members. The apparently terminal 
position of the rudiment from which they are derived is due simply to 
the arrest of the growing-point. Though commonly formed later than the 
cotyledons, it is terminal from the first. Forms such as Hypecounc , in 
which the growing-point appears at the same time as the cotyledons, or 
earlier, represent the primitive arrangement. The single cotyledon of 
Monocotyledons is, however, truly terminal, and cannot be treated as 
homologous with one or both cotyledons in Dicotyledons. It is an organ 
sui generis , and can never under any circumstances be considered as a leaf. 
Its leaf-like appearance in some seedlings is due to homoplastic adaptation 
(Balfour, 8, pp. 827-8 ; cf. also Coulter and Chamberlain, 19 , p. 208). 
We need not consider this hypothesis at greater length, since it is 
hardly compatible with the monophyletic origin of Angiosperms. Indeed 
its supporters commonly assume a polyphyletic origin. 
The second school is represented among recent writers by Professor 
H. L. Lyon ( 57 , 58 ). He considers the single cotyledon of Mono- 
cotyledons as truly terminal, and both cotyledons of Dicotyledons as 
derived from it by fission. This hypothesis he has carried out to its logical 
consequences. His views are founded on the structure of the embryo in 
Nelumbium. He has shown that in N. luteum the embryo has no suspensor, 
and becomes a large, undifferentiated, nearly spherical mass before the 
cotyledons are indicated. They appear as a crescent-shaped ridge, which 
