Trends in the evolution of the angiosperm flowers

Abstract

The flower is classically defined as a determinate shoot that bears modified leaves. It is believed that the primitive flowers are those which possess numerous floral parts, all spirally arranged on the floral axis. Discussing the origin of the flowers it is claimed that the flowering plants might have evolved from the hypothetical ‘Proangiosperms’ that existed in the Late Mesozoic Era and had characters of both gymnosperms and angiosperms. Among other factors, seasonal climate is believed to have caused the angiospermatization in the early proangisperms.

Primitive flowers, such as those of Magnoliales, are all solitary, regular and moderately sized in which the floral parts are all separate and indefinite in number. It is stressed that the evolution of the flower is guided by (a) principle of progressive economy in the production of floral parts, and (b) principle of progressive adaptation to insect visitors. Adaptations of angiosperms to insect visitors include development of conspicuous flowers (either by enlargement of individual flowers or by aggregation of separate flowers), development of zygomorphy and fusion of floral parts. Primitive flowers lack a differentiation between calyx and corolla. Derived perianth features are said to include (a) evolution of two distinct whorls of perianth-calyx and corolla, (b) establishment of a regular number of floral parts, (c) fusion of sepals and petals, and (d) development of zygomorphy.

Primitive stamens, characteristics of the genus Degeneria (Degeneriaceae: Magnoliidae) are broad and laminar and evolution proceeded towards transformation of such laminar stamens to a distinct filament and anther. Tendency towards reduction in the number of stamens is also quite evident. Similarly the flowers with many free carpels (as in Magnoliales) have given rise to flowers with syncarpous gynoecia which have further evolved to give rise to inferior ovary by the fusion of gynoecial walls to the calyx and receptacle.

Finally it is said that the above evolutionary modifications in the flowers/flower parts are fundamentally guided by the mode of pollination and pollination mechanism and have been attempted independently by different groups of plants- a fact that suggests the polyphyletic nature of angiosperms.