Across flowering plants, many lineages have transitioned from radially symmetric flowers (with multiple planes of symmetry) to bilaterally symmetric flowers (with a single plane of symmetry). Several hypotheses have been proposed to explain this trend, including the “pollen placement hypothesis”. This hypothesis reasons that a reduction in the number of planes of floral symmetry should result in more precise placement of pollen (and stigmas) onto pollinators as a result of more consistent pollinator entry into flowers. The resulting improvement in pollen-transfer efficiency should increase the plant’s potential reproductive output per unit floral reward produced.
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| Bombus impatiens visiting a real bilaterally symmetric flower - a snapdragon |
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