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WIND POLLINATION IN THE ANGIOSPERMS: EVOLUTIONARY AND ENVIRONMENTAL CONSIDERATIONS

1969; Oxford University Press; Volume: 23; Issue: 1 Linguagem: Inglês

10.1111/j.1558-5646.1969.tb03490.x

1558-5646

Donald R. Whitehead,

Plant Parasitism and Resistance

Seed plants have evolved a vast array of pollination mechanisms, some involving fascinating interactions between animal vectors and highly specialized flowers, others essentially abiotic, utilizing either wind or water as a dispersal agent. Among the gymnosperms wind pollination is primary, although biotic systems have evolved in a number of groups. In contrast, anemophily has evolved secondarily in the angiosperms; the primitive angiosperms and their immediate ancestors were zoogamous, possibly beetle pollinated (van der Pijl, 1960). Many of the evolutionary changes leading to the angiosperm flower increased the probability of pollination by various animal vectors while at the same time decreasing the efficiency of abiotic pollination (e.g., enclosure of ovule, shortening of floral axis, enclosure of androecium and gynoecium by perianth). Anemophily is thus a derived condition in the angiosperms. Yet, despite its essentially random and inefficient nature, depending upon large numbers of pollen grains and the vagaries of atmospheric circulation, it has arisen independently in several angiosperm families. In fact, in some families (e.g., Gramineae) and in some environments (northern deciduous forests) it is the dominant means of pollination. The occurrence of wind pollination appears to be related to certain environmental parameters; for example: latitude, species diversity, vegetational structure, the deciduous habit, and insular environments. It is notoriously infrequent in the tropical rain forest and becomes progressively more significant with increasing latitude. Similarly, it appears to be more common in areas that are less diverse floristically, and it is often dominant in vegetation characterized by open structure (e.g., savanna) or seasonal loss of leaves (northern temperate forests). The frequency of wind-pollinated species is high in island floras (e.g., Hawaii, Carlquist, 1966). The purpose of the present paper is to review certain aspects of wind dissemination, to reflect on the factors governing the present geographical distribution of anemophily, and to speculate on its probable evolution within the angiosperms. Much insight can be provided by an analysis of theoretical aspects of dispersal and capture of pollen-sized particles. In fact, the distribution and evolution of the anemophilous condition can best be understood if aspects of pollen release, transfer, and capture are first considered.