A great deal of a flowering plant life is left to the discretion of pollinators. These indeed do play a very fundamental role in plant mating. Not to say, they are often responsible for a sensible amount of selfing (self-fertilization) and outcrossing (mating with an unrelated plant of the same species), depending on their behavior once they are landing on a flower.
The Velvet Leaf (A. theophrasti) has gorgeous yellow flowers, which open almost successively on the plant, thus reducing the opportunity for geitonogamous pollination (self-pollination occuring between two flowers from the same plant, usually mediated by pollinators).
While this way of flowering is convenient to naturally decrease selfing, pollinators themselves sometimes contribute their own way to an improved cross-fertilization (outcrossing).
Let’s see how this happens… First, our halictid bee is landing on the stigma of the flower, next to the stamens born from the style (just like in any good Mallow flower) and begins to feed on available pollen. We can see a rather active tongue (oops, let’s not anthropomorphize too far; it’s a proboscis in lieu of a real tongue of course!).
While doing this, the bee is depositing pollen from the previously visited flower onto the hairy stigma heads.
Than, our little bee is grooming a little bit, removing pollen from its thorax hairs. This will result in leaving free place for the pollen of the current flower as soon as it will go for nectar, the very next step of the visit.
As you can see, it is also passing by the next stamens and gathering some more pollen…
Yep, that’s what happens really next: the bee’s walking the stigma down to the corolla where nectaries at the very base of petals are offering pollinators some exquisite refreshment.
A next table round is thus the last plant gift to the insect. It’s rather impressive that grooming takes place some time after landing on the stigma and therefore after cross-pollen deposition occurred, and before the bee is taking some more pollen from the current flower.
This probably results in an increased cross-pollination, for the next flower to be visited is very likely to belong to another plant.
Amazingly, the bees I observed pollinating the Velvet Leaf plants at my (ex-)door did behave quite similarly…
This system requires only two plants to reduce selfing efficiently, a condition that should occur quite frequently in natura. Does it have anything to do with the apparent success of A. theophrasti as an invasive species?