Aha. It turns out the mystery plant that I thought at first might be a form of bluebonnet may be “wild alfalfa.” I sent a picture to the Lady Bird Johnson Wildflower Center in Austin and got this reply.

Our expert, Joe Marcus, believes your “bluebonnets” are really wild alfalfa or scurfy pea, Psoralidium tenuiflorium. It is in the same family as bluebonnets, the bean/pea family (Family Fabaceae). Here are a couple of links for photos and more information:

http://wildflower.utexas.edu/plants/result.php?id_plant=PSTE5

http://www.lib.ksu.edu/wildflower/wildalfalfa.html

I believe the expert is right. Thanks to “Mr Smartyplants, ” who is really Dr. Nan Hampton, for the help. Isn’t it great to have these kind of resources available?