I want to understand the four mechanisms of pollination in papilionaceous flowers: pump/piston, explosive, valvular and brush.
The topic is lightly touched upon in this part of a Wikipedia article, but I find the explanations somewhat shallow and incomplete.
This study from the journal Plant Systematics and Evolution about the explosive mechanism mentions all mechanisms but only deals with the explosive. Regarding the explosive mechanism it mentions that the androecium and gynoecium are subjected to pressure from the keel. My interpretation is that when a pollinator presses upon the keel the reproductive column "snaps" up due to the released pressure. Taxa which I know utilize this mechanism are: C. scoparius, Medicago sativa and the three Desmodium species studied in aforementioned publication (D. incanum, D. subsericium and D. uncinatum) but due to my location I have not been able to study the explosive mechanism in the field. If my interpretation is correct, then how is the pollination transferred to the pollinator after the column is released? In the previously mentioned Wikipedia article it is said that the column hits the banner (standard), but it does not tell whether this "collision" with the banner is what causes the pollen to be released. Does the collision with the banner release the pollen in a "cloud" which envelops the pollinator? Or does the reproductive column hit the pollinator along the trajectory it travels from keel to banner?
The pump/piston mechanism I have on the other hand been able to study quite easily due to the great amount of Lupinus polyphyllus growing in my area which inhibit this technique of pollination.
This study published in The Scientific World Journal explains the valvular mechanism as follows:
In the valvular mechanism, the upper rim of the keel is unsealed; it opens along its total length when the keel is moved downwards by the pollinator and closes when the insect leaves the flower. In this case pollen can be released repeatedly to numerous visitors.
and the brush mechanism in this way:
In the brush mechanism, as the pistil is longer than the stamens, the stigma extends beyond the anthers, avoiding self-pollination; the upper part of the style develops erect trichomes acting as a pollen brush. When sufficient pressure is exerted on the standard and wing petals, the pistil protrudes from the keel tip: the stigma comes first into contact with the insect receiving (if present) external pollen and the style brushes the pollen on the visitor. When pressure is released, style and stigma return to their former position inside the keel.
As for this last explanation of the brush mechanism I do not entirely understand how the spread of pollen occurs. The publication says that the stigma extends further than the anthers and that the "style brushes the pollen on the [pollinator]". What do they mean by this? Does it have to do with the trichomes on the style? If so, how do they play into the transfer of pollen?