Aristolochia arborea, perhaps. Here is a link that might be useful: images

It sounds like a rain lily (Cooperia pedunculata). They only bloom after a rainfall. After they flower, they produce small black seeds w/ a paper-like coating that helps them float in the wind. The seeds sprout and create a bulb that stays dormant until the next rainfall causes it to bloom. The white variety seems to be the most common. I've also seen yellow and pink varieties. If you let them flower and go to seed, you can have a nice little grouping in a few years. One of those lovely surprises after a good rain. :-) Here is a link that might be useful: Hill Country Rain Lily

coatfetish, that's a nice crabapple, I can smell it's fragrance all the way up here. I have a way to get the tree you want, take cuttings. This pic shows rooted cuttings of my 'Tina' crab. Knock on your neighbor's door and ask for cuttings. Do this around the end of May or anytime in June. The medium I use is 50/50 peat-perlite. I also use a rooting hormone. I put mine under mist and they are rooted within a month. Since you probably won't have access to mist you can create a propagation chamber, this has been discussed many times here. You could also use a cold frame, out of the sunlight and keep the top closed. PS, if you do this, don't take 5 or 6, take 100. That way you can keep the strongest ones for yourself and give away the rest.

Thanks, floral_uk. That seems like a strong possibility, though none of the flowers I'm looking at seem to open as much as Cardamine's flowers. Maybe I need to give them a few days. The plants are about 9" tall which is about the same for Cardamine. I see this Cardamine is sometimes called "Sand bittercress" which would accord with these plants growing in virtual gravel. Here are several photos I just took (some of which are virtual duplicates but they look beautiful!) I'll get a good photo of the basal leaves later today.