Patterns in the evolution of amentiferous fruits

Abstract

The diversity of fruit types within the Amentiferae appears to be the result of different seed dispersal and seedling establishment strategies that have evolved independently in the amentiferous families within the confines imposed by the wind-pollination syndrome. Dispersal of unadorned fruits and seeds by wind (e.g.,Betula, Rhoiptelea, Casuarina) has preceded the development of more efficient air-borne devices (e.g.,Ostrya, Carpinus, Engelhardia). Animal dispersal is the most advanced strategy, relying rarely on drupes, as in the shrubby Myricaceae, or on nuts, as inCorylus, Quercus, Carya, Juglans, Alfaroa, etc. The pattern of seedling establishment shows a structural-functional relationship to the seeddispersal strategy. Epigeal germination predominates in plants of open habitats. Those species with small fruits rely on wind dispersal, and their seeds have a relatively short germination time, whereas species with medium-sized fruits are often animal-dispersed with seeds that may have protracted germination times. In either case the reserve food supply for the young seedling is limited, and light is needed to spark photosynthesis. Hypogeal germination has evolved independently in several amentiferous families (e.g., Betulaceae, Fagaceae, Juglandaceae). This pattern is associated with closed habitats and plants with large seeds that have the capability of establishing vigorous seedlings in microhabitats of reduced light intensity where photosynthesis is impaired. The subterranean protection afforded the cotyledons and axillary meristems is a correlative feature that may have considerable importance in seedling survival where desiccation and predation are intense.