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Raceme, untripped, and tripped flowers.)
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On the left is an open flower that has not yet been visited or pollinated by a bee. Before pollination, the column, which consists of the anthers and the style, is held in place under pressure between the lower keel petals. When a bee lands on the keel, the column is released and pops up forcefully and hits the bee in the head, scattering pollen from this flower. At the same time, the stigma picks up cross-pollen on the bee. This interaction is known as “tripping”. In alfalfa it’s irreversible. The flower looks different, and the bee can see the difference.
Honey bees avoid landing on the keel and tripping the flower; apparently to avoid getting hit in the head by the column. Rather, they collect nectar by sticking their tongues in the base of the flower from the side of the keel. When they do this, the flower is not tripped. Thus, honey bees don’t pollinate alfalfa well.
Flowers wilt within hours when they are tripped, but will stay open for about a week if they are not tripped. This is important in the dynamics of alfalfa pollination, as my research has shown (see slide show Part 2)..
So, there is essentially one chance for the flower to get cross pollinated, and one chance for the bee to collect pollen and nectar from the flower.