While visiting Waterloo State Recreation Area near Chelsea, Michigan, I came across quite a bit of pickerelweed (Pontederia cordata, Pontederiaceae). These semi-aquatic plants were growing thick just offshore, emerging from several inches of water. Pickerelweed establishes a dense network of rhizomes in the inundated soil. Every spring new shoots, leaves, and flowers grow from these roots.
To me the most interesting thing about pickerelweed is that it’s tristylous (Price and Barrett 1982). Various mechanisms to prevent self-fertilization have evolved in flowering plants, and tristyly is a relatively rare one. A given plant can grow one of three genetically-determined flower structures (morphs), each with its own dimensions of the various reproductive organs. Bumble bees and other insects visit the flowers for the nectar and unwittingly transfer pollen from one flower to another. Because of the structural differences in the flowers, pollen from a given morph is unlikely to be successfully deposited on other flowers of the same morph. Since one plant has only one flower morph, it is therefore unlikely to fertilize itself (and others that happen to be like it). It is capable, however, of fertilizing plants with the other two morphs. With pickerelweed this trait has been found to be a stable method for promoting self-incompatibility and vigorous outcrossing (Price and Barrett 1982).
Price, S.D. and S.С.H. Barrett. 1982. Tristyly in Pontederia cordata (Pontederiaceae). Canadian Journal of Botany 60:897-905.