| Summary: | The genus Philonotis (Musci) is revised for North /America, north of Mexico, and exclusive of Greenland, following the last revision for this region by Seville Flowers in 1935. Collections and field observations of all but two of the recognized taxa were made from a wide diversity of habitats throughout most of their geographic range in North America. Numerous collections were cultured in a uniform environment to determine the stability of taxonomically diagnostic morphological characters. The position of papillae on the leaf cells, the structure of marginal cells, and to some degree the leaf shape and leaf cell shape are stable characters that are not modified by the environment. All other morphological characters examined are ecophenic and cannot be utilized to segregate taxa. The present revision recognizes seven species and two varieties; Philonotis glaucescens (Hornsch.) Broth., P. longiseta (Rich, in Michx.) Britt., P. sphaerocarpa (Hedw.) Brid., P. mavohioa (Hedw.) Brid., P. capillaris Lindb. ex Hartm., P. fontana (Hedw.) Brid. var. fontana, P. fontana var. amevicana (Dism.) Flow, ex Crum, P. fontana var. pumila (Turn.) Brid., and P. yezoana Besch. et Card, in Card, (new to North America). Chromosome counts of n = 6 for P. glaucescens and n = 12 for P. longiseta are new reports. Keys, descriptions, illustrations, and distribution maps are provided for identification of the species. Science, Faculty of Botany, Department of Graduate
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