The taxonomic status of Mimulus glabratus var. michiganensis (Pennell) Fassett.

There are two purported varieties of Mimulus glabratus in Michigan. Both commonly grow in groups of fifty to several hundred individuals in shallow water along streams and lake shores. One is the widespread M. glabratus var. fremontii which ranges from Ontario to Manitoba and south to Mexico. The other is M. glabratus var. michiganensis, a Michigan endemic found only in the Mackinac Straits region in Cheboygan, Mackinac, Charlevoix and Emmet Counties. Pennel (1935) and Fassett (1939) accorded the later varietal recognition due to its larger flower, more sinuate-dentate leaves, unspotted corolla, and a more upright habit than M. glabratus var. fremontii. However, an examination of herbarium specimens by Crispin (personal communication) and myself suggests that a continuum of morphological diversity between the two varieties may exist. There are specimens annotated by Pennell as M. glabratus var. fremontii whose leaf margins are as sinuate-dentate as those of M. glabratus var. michiganensis. Other specimens labelled as M. glabratus var. michiganensis are noted to have had slightly spotted corollas and McArthur et. al. (1972) reports the same chromosome number for both varieties (n=15). At present, M. glabratus var. michiganensis is listed as threatened by the state of Michigan, however, it is not considered threatened by the Federal Endangered Species Office because of its uncertain taxonomic status. The purpose of this investigation was to determine whether M. glabratus var. michiganensis is indeed a distinct variety, or simply a morphological extreme of the widespread M. glabratus var. fremontii.