HYPNACEAE 85 



Stem leaves notably different in most species, larger and pro- 

 portionately broader, often more slenderly acuminate and less 

 strongly serrate; seta smooth to very rough, twisted to the right, 

 capsules short-ovoid to oblong-ovoid, cernuous-arcuate and 2-4: 

 1 (cylindric in B. oxycladon and nearly erect and symmetric in 

 B. acuminatum and B. cyrtophyllum) , slightly contracted under 

 the mouth when dry and empty, operculum conic to short 

 rostrate; peristome characteristically hypnaceous and perfect 

 except in the species with erect capsules. When in fruit this 

 genus is easily recognized in most cases by the short, thick, 

 dark-colored capsules which are usually unsymmetric and hori- 

 zontal; spores almost without exception maturing in late autumn 

 or early winter. 



The grouping of species given below is a little different from 

 that given in my monograph of the genus, in Vol. VI of the 

 Memoirs of the Torrey Botanical Club, but represents opinions 

 based on a great deal of additional study. 



(1) The Salebrosum group consisting of B. salebrosum, B. 

 flexicaule, B. acutum, B. campestre, B. oxycladon, and B. digas- 

 trum. 



This group is characterized by smooth seta (except B. cam- 

 pestre), leaves plicate or sulcate when dry (except B. acutum), 

 slenderly acuminate, concave and excavate at basal angles with 

 the margins turned back parallel to the plane of the leaf (or 

 reflexed in the ordinary descriptive language) in the lower por- 

 tion; basal cells broader and shorter, quadrate alar cells numer- 

 ous, hyaline or slightly chlorophyllose, stem-leaves ovate-lance- 

 olate; annulus poorly developed, fairly well developed in B. 

 acutum. 



(2) The Rutabulum group including B. rutabulum, B. rivulare, 

 and B. Starkei,, characterized by rough setae, well-developed 

 annulus, ovate-deltoid stem leaves, all leaves less concave and 

 alar cells less numerous, except in B. rivulare. 



(3) The Acuminatum group resembling the salebrosum group 

 in gametophyte characters but with nearly erect symmetric 

 capsules and peristomes without cilia, consisting of B. acumin- 

 atum, B. cyrtophyllum and some southern species. 



(4) The Plumosum group including B. plumosum and B. 

 populeum, characterized by rough setae and densely chloro- 

 phyllose quadrate alar cells. 



