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Wind Pollination in the Angiosperms: Evolutionary and Environmental Considerations
Donald R. Whitehead
Vol. 23, No. 1 (Mar., 1969), pp. 28-35
Published by: Society for the Study of Evolution
Stable URL: http://www.jstor.org/stable/2406479
Page Count: 8
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Studies of the aerodynamics of particle transport and capture suggest that the conditions most propitious for effective wind pollination are as follows: (1) production and release of large numbers of grains; (2) both anthers and stigma exposed; (3) grains falling within a certain size range (20-40μ); very small grains will be dispersed readily but cannot be captured efficiently by the stigma, large grains have a high terminal velocity, hence will settle too rapidly; (4) exine of grains thin (or with large air spaces), sculpture smooth; (5) the stigma should present much surface area to capture grains, but, since collection efficiency will decrease with increasing stigma diameter, the increase in surface should be accomplished through the evolution of a complex stigma with many branches of small diameter; (6) individuals of the species should not be too widely spaced in the vegetation; (7) the vegetation should be open in structure or deciduous; thus there will be few obstructions to transport during at least one portion of the year; (8) flowering must be closely coordinated by relatively unambiguous environmental stimuli; (9) pollen release (hence flowering) should coincide with the most favorable time of the year for transport (low probability of precipitation, adequate winds and turbulence, deciduous season). The geography of wind pollination in the angiosperms can be understood best in terms of these prerequisites for efficient transport of pollen. Anemophily is very infrequent in the tropical rain forest, becomes more frequent as one moves to more seasonally variable environments, and is frequent in northern temperate forests. The environment of the tropical rain forest is unsuitable for anemophily: (1) species diversity is very high, hence individuals of the same species are apt to be widely spaced; (2) the forest is densely structured, hence wind velocities are low and there are many obstacles to transport; (3) there is no leafless season; (4) rainfall is frequent throughout the year, hence transport will be limited; (5) there are few unambiguous stimuli which can coordinate flowering; (6) potential animal pollinators are abundant. These conditions do not exist in northern temperate forests, or in prairies, or in savannas. It is possible that the evolution of wind pollination in the angiosperms paralleled the evolution of the deciduous habit. Axelrod has suggested that the deciduous habit evolved in response to seasonal drought as angiosperms migrated into lower middle latitudes during the early Cretaceous. Both the deciduous habit and the physical conditions existing just peripheral to the tropics would favor the evolution of wind pollination. An examination of seasonal environments in the tropics at present and the fossil record of deciduous and wind pollinated angiosperms supports this contention.
Evolution © 1969 Society for the Study of Evolution