I took a walk between downpours and sprinkles and collected a bunch of "weeds." It turns out Bermuda Buttercup, (Oxalidaceae) Oxalis pes-caprae, is a rare plant in South Africa. It's a hugely successful weed here. I took some photos of the pistil and stamens and noted curious structures. An image search turns up all sorts of discussions about the pistil and stamens due to the curious reproductive choices of the Oxalis plants: despite having both male and female structures, they cannot self pollinate. There are three different arrangements and cross pollination occurs with a morph of a different arrangement. Apparently it was very rare for the California plants to set seed -- i assume this is because the populations of morphs were not balanced. It turns out there's an article looking at this question in respect to the invasive populations in the Mediterranean.
Also, in more Plants are REALLY WEIRD commentary, not only do the morphs not self pollinate, but one morph has FIVE sets of chromosomes while another has THREE.
We're more closely related to slime molds and fungi than we are to plants.
So, we now have a mystery: why is a plant that can't set seed invasive? Apparently the little bulbs it set are very successful.