There were a lot of people who added LaPlante as DIT names (nicknames unrelated to their original family name). Some people dropped it after the first generation and others adopted the DIT name and dropped the original name after a generation or so. My family has LaPlante DIT names and Causleys also.

You could look at the newsletter of the disbanded Plante Family Association. They were in Canada and much of the information is in French. There is an article about the LaPlante name at http://pages.infinit.net/plante2/vol10no4/vol10no4e.htm

Another fantastic source is Paul A. Plante who has information about over 30,000 Plante, LaPlante and Plant related names.