The plants are members of the pea family, or Fabaceae, and the genus name is Mimosa. There are several species. Another of them is also common in this area, although I haven't seen any yet this year. Although the leaves and flower of this second plant are quite similar, the plant is much smaller, and it is usually called the sensitive plant, from its habit of rapidly collapsing its leaflets when they are touched. You can see a Quicktime movie of this collapse here.
Members of the pea family are as important as any other group of plants, because they work, in conjunction with bacteria in lumps, or nodules, on their roots, to make Nitrogen available to themselves, to animals that eat them, and to the soil when they die. (See here for another post on a member of this family, with some reference links)
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