It is called heliotropism. There has been much written on the subject. Do a search using that word. (Despite the similarity in name, it has nothing to do with the plant commonly called "Heliotrope".) "Tilman Lamparter" <lamparte at zedat.fu-berlin.de> wrote in message news:39ABE9C9.D6EC35BA at zedat.fu-berlin.de... > >