Pollination of Aconitum (Ranunculaceae) - Monkshood
A genus of c. 250 spp. throughout temperate regions of the northern hemisphere.
In these remarkable hooded flowers the sepals are petaloid, generally blue or violet (sometimes yellow), and attract bumblebees (Bombus spp.). Only bumblebees can pollinate the flowers and Aconitum can only reproduce effectively where bumblebees are present.
The top sepal is in the form of a hood (galeate; galea = helmet) which serves to protect the reproductive organs from rain and probably to exclude visits from non-pollinating insects. The space formed by the sepals (and in particular the hood) fits the bumblebee body quite precisely.
The petals take the form of curved tubular nectaries of variable and complex shape that prevent short-tongued insects from accessing nectar while being suited to bumblebee foraging. To access the nectaries bumblebees have to crawl over the central boss of reproductive organs, pollination therefore being effected by the ventral side of the insect.
The flowers are protandrous with numerous stamens and (3-5) free carpels. Immature stamens are directed downwards and are therefore kept away from the body of the visiting insect. When mature the stamens are erect and pollen brushes onto the underneath of the insect. Old stamens again become directed downwards and out of the way. When all the stamens are finished the carpels enlarge and visiting insects bringing pollen brush against them on entering the flower, self-pollination thereby being effectively avoided.