Observations on Fusion and Evolutionary Variability in the Angiosperm Flower

Abstract

Fusion in the angiosperm flower is regarded as progressive only to the degree dic- tated by structural factors, and it is subject to parallelisms and reversals. Fused parts are not seen as basically more competitive than equally specialized unfused parts, but fusion increases the options for evolutionary variability toward increased, reduced, or more integrated fusion of parts, and this is expressed in a more diverse progeny with more options for survival. The various degrees of fusion between the parts of the angiosperm flower are a major fac- tor in flowering plant classification. Increase in the amount of fusion in floral parts has been placed logically in a progressive series, though such a progression has never been adequately tested. Furthermore, the more fused condition in flowers has been dimly perceived as bene- ficial, though the actual benefits are not well understood. In an attempt to improve under- standing and gain a more realistic concept of this evolutionarily and taxonomically impor- tant condition in the flower, the present author offers the following results of various obser- vations and discussions regarding fused floral structures. The question of benefits of floral fu- sion is reviewed with special consideration of the form of fusion involved in the inferior ovary. The discussion of fusion in the angiosperm flower concentrates here on three aspects: 1) apparent progression in fusion from seemingly separate parts to highly fused types; 2) special qualities found in fused parts; and 3) the na- ture of benefits derived from fusion of parts. The present considerations exclude the type of fusion that occurs in such structures as the an- thers of the Asteraceae or the tips of the carpels in some Apocynaceae where little or nothing more than fusion of the cuticle is involved.