Wind and Water: Abiotic Pollination

This chapter examines abiotic pollination via wind or water. It begins with a discussion of wind pollination or anemophily, which is a common occurrence in modern plants, especially in most gymnosperms, in the catkin-bearing angiosperm trees, and in cereals (Poaceae and Cyperaceae). Anemophily is predominantly a derived condition in angiosperms, associated with ecological conditions where zoophily is difficult. Transition from zoophily to anemophily has occurred at least sixty-five times in such circumstances. The chapter looks at wind-pollinated angiosperm and conifer flowers and goes on to consider critical factors affecting pollen movement in anemophilous plants. It concludes with an overview of hydrophily, or water pollination.

Abiotic pollination: an evolutionary escape for animal-pollinated angiosperms

Early botanists considered abiotic pollination to be primitive in angiosperms. But we now deduce from studies of palaeoecology and of extant ‘prim itive’ angiosperms that animal pollination was concom itant with the rise of the angiosperms. Recent studies of wind and w ater pollination in angiosperms also show these systems to be highly sophisticated. If entomophily contributed to the rise of the early angiosperms, why should many of their descendants have later evolved abiotic pollination systems? Although entomophily was initially advantageous to the early angiosperms, abiotic pollination systems may be superior in areas of low species diversity, newly colonized habitats, and places with extremely short growing seasons or other adverse climatic conditions. Abiotically pollinated plants are not constrained by the range of animal pollinators, and as a result are spectacularly successful in long-distance dispersal. Abiotic pollination also offers an escape from deleterious sexual selection and from dependency on pollinators that are climatically limited in their distribution in space or time and vulnerable to extinction. Because evolution of abiotic pollination frequently leads to dicliny or dichogamy, it is largely irreversible. This evolutionary irreversibility coupled with lowered rates of extinction and speciation give wind- or water-pollinated taxa unique phylogenetic profiles. As a large quantity of pollen is wasted by anemophilous plants, it is surprising that so many vigorous species of this kind abounding with individuals should still exist in any part of the w orld; for if they had been rendered entomophilous, their pollen would have been transported by the aid of the senses and appetites of insects with incomparably greater safety than by the w ind... It seems at first sight a still more surprising fact that plants, after having been once rendered entomophilous, should ever have again become anemophilous. (Darwin 1876, p. 407)