So I missed a blossom on another adjacent Ipomoea aquatica and I actually have two open this morning!
I emasculated the first carefully after opening it an observing the stamens had not contacted the stigma and then pollinated it with the pollen of Ipomoea fistulosa.
I did not emasculate the second blossom, I just pushed the pollen covered paintbrush right into the flower to touch the stigma, the part that the pollen contacts and germinates on, before it grows a pollen tube, which issue gametes to achieve (double) fertilization of the eggs in the carpels of the ovary at the base of the flower.
Each carpel in a fruit can contain one seed and has two eggs. Ipomoea species don't have many carpels but these species open this morning typically have 5, so that is theoretically 5 potential chances per blossom to set a single seed.
