The early ontogeny of the female inflorescences of Fagus sylvatica and Costarica satva is described together with some other features of the inflorescences and flowering shoots.
The mode of ontogeny of the inflorescences in these species suggests that the two types of cupule are exactly comparable, and that the segments of the cupules (free in Fagus, united in Castanea) are referable to the normal cymose branching of the inflorescence. This is also supported by the vascular organization in the cupules of these species and of Nothofagus. It is, therefore, concluded that the cupule segments in these genera are sterile axes of the next higher order in the dichasium above the flowers they enclose. Thus a single flower is enclosed in a cupule consisting of only two segments (Nothofagus, Castanea), three flowers in a cupule of four segments (Fagus with the primary flower absent, Nothofagus, Castanea), and so on.
It has been possible to apply this interpretation to the various forms of cupule occurring throughout the Fagaceae, and by so doing certain important evolutionary trends have been recognized which have suggested a more natural basis for classification and the need to re-define some genera.
The status of the cupule of Fagus as the primitive type is upheld by a general lack of specialization in this genus as regards secondary xylem, cotyledons, some floral characters, and the manner in which the inflorescences are borne. Geographical distribution is briefly considered in this connection.
General phylogenetic results of this analysis are expressed in the following proposed subdivision of the Fagaceae:
Subfamily I Fagoideae
Subfamily II Castaneoideae