from The Century Dictionary and Cyclopedia
- Those petals in a papilionaceous flower which unite to form the keel.
Sorry, no etymologies found.
But I have forgotten to say that my maximum difficulty is trees having papilionaceous flowers: some of them, I know, have their keel-petals expanded when ready for fertilisation; but Bentham does not believe that this is general: nevertheless, on principle of nature not lying, I suspect that this will turn out so, or that they are eminently sought by bees dusted with pollen.
Yet I have seen humble-bees whilst sucking the nectar depress the keel-petals, and become so thickly dusted with pollen, that some could hardly fail to be left on the stigma of the next flower which was visited.