Dec. 22, 2017

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Water hemp is a study in applied aerodynamics.  It's wind pollinated.  With such plants, there's more to this than just sticking your stigmas up in the air and hoping for the best.  Studies have shown that wind pollinated plants have sophisticated strategies for scattering, collecting and targetting pollen.  The male flowers are shown here.  These flowering branches are leafless, and the anthers stick out well beyond the reduced perianth.  This enables the wind to reach the anthers at maximum effect.  I don't know about this species, but the pollen of many such plants has air pockets.  These make the grains much less dense and easily affected by air flow.  In contrast, the stem with female flowers shown on the wildflower page is leafy.  Those leaves break up the air flow, slowing down the pollen and creating eddies around the flowers.  The feathery stigmas can then comb the pollen from the air.  Wind tunnel studies of some plants have shown that such eddies are specially directed to take pollen to its target.  Wind pollination was the first method that evolved.  Pollination by other vectors is more recent.  But some newer plants, such as ragweed, have again adapted to using the wind.  Aren't we glad!