de Fraine.—The Seedling Structure of certain Cactaceae. 171 
time was urgent, one seed-leaf delayed its appearance more and more until 
finally the symmetry of the root-stele of the seedling had to be attained 
without its aid, and the second cotyledon became practically the first leaf. 
We may consider that, since the cotyledons originally were equivalent in 
function, their vascular structure would be similar, each of them would 
contribute equally to the root-structure, and each would furnish a strand 
opening out somewhere in its course as a ‘ double 5 bundle. In the ‘ sucking ’ 
cotyledon it would obviously be an advantage to delay the closing up of 
the ‘ double ’ bundle and to separate its parts, producing the appearance of 
two distinct bundles, especially where much endosperm had to be absorbed. 
In these cases the structure of the first cotyledon and the second would be 
different, but the second might still, although its appearance was delayed, 
contribute to the root formation. Finally, when the second seed-leaf did 
not develop until very late, the first would necessarily carry out the 
transition unaided ; its two strands, formed as indicated above, would now 
each act as a ‘ double ’ bundle, and the difference between the two originally 
equivalent cotyledons would be still further increased. In this connexion 
it is important to remember that ‘ seedlings are thrown on their own re¬ 
sources at so early a period in their life history that the struggle for 
existence, repeated through many generations, often transforms their whole 
structure. At the time of germination this structure is still so little 
differentiated as to be extraordinarily plastic.’ 1 
It is, therefore, quite conceivable that the difference in structure between 
the cotyledon and the so-called ‘first leaf’ in Monocotyledons can be 
explained as due to the physiological needs of the young plant, and 
if so, A. W. Hill’s theory is still left without any serious objection 
against it. 
In conclusion, of the three hypotheses which have been put forward in 
explanation of the evolution of Monocotyledons from a dicotyledonous 
ancestry :— 
1. The fusion hypothesis is seriously weakened by the discovery of 
the facts described above. 
a. The suppression hypothesis is but slightly affected. 
3. The first leaf hypothesis is supported, for the chief objection 
which has been urged against it has been shown to be no longer valid. 
In regard to the transition-phenomena of the seedlings of the Cactaceae, 
much variation is to be found throughout the group ; it does not always 
appear to be uniform even in the members of a single genus, as is seen in 
Opuntia and Mamillaria , although it may be fairly constant, as for example 
in Cereus. Ganong 2 has already pointed out that in all probability the 
adaptations to physiological needs in the adults have worked back into the 
1 Sargant, E.: The Origin of the Seed-Leaf in Monocotyledons. New Phyt., i, p. 108. 
2 Ganong, W. F. : loc. cit. 
