94 HYPNACEAE 



Key 



i — Paraphyllia abundant, alar cells inflated filicinuni 



Paraphyllia few or wanting, alar cells not distinctly inflated 2 



2 — Costa percurrent or nearly so, or excurrent 3 



Costa ending well below the apex, usually not far above the middle . . 6 

 3 — Stem leaves entire, cordate-ovate to oblong-lanceolate, apex rather 



blunt ; all forms aquatic ,5 



Stem leaves narrower in outline, usually acute with a more slender 



acumen; growing in water or wet places 4 



4 — Aquatic, submerged except at low water; basal leaf cells usually 



plainly enlarged; costa very stout irrignum 



Growing in wet places but not often submerged; basal cells much less 

 differentiated; costa more slender and apex usually more slenderly 



acuminate varium 



5 — Stem leaves oblong-ovate to broadly oblong-lanceolate, 1-2 mm. in 

 length, gradually and evenly narrowed to a rather blunt apex. 



fiuviatile 

 Stem leaves ovate to cordate-ovate, usually less than 1.2 mm. long, 



more abruptly narrowed to a distinct acumination orthocladon 



6 — Median leaf cells reaching io: I or longer riparium 



Median leaf cells less than 8 : I io 



IO — Leaves widely spreading when dry; marginal cells at base of leaf 



oblong to rectangular 1 1 



Leaves not widely spreading; marginal cells at base quadrate or 



transversely elongated serpens 



II — Stem leaves i mm. in length or less, rarely more, ovate-lanceolate. 



Juratzkanum 

 Stem leaves more than I mm. in length; leaves broadly ovate-lance- 

 olate to cordate-ovate Kochii 



The genus as here constituted divides itself pretty clearly 

 into three subgenera. 



Euamblystegium Broth., of which A. serpens is the center. 

 Slender plants, not aquatic; leaf cells 2-6: I, rarely longer; costa 

 slender, reaching to the middle of the leaf or a little farther (to 

 the apex in com pactum). 



This group includes serpens, Juratzkanum, Kochii and com- 

 pactum. Kochii is usually put with the next group, but its 

 nearness to Juratzkanum and its wide leaf cells convince me that 

 it belongs here. 



Leptodictyum Schimp. contains A. riparium and its allied 

 vacillans. The group is characterized by a generally aquatic 

 or subaquatic habitat, long, narrow leaf cells, 8: I, sometimes 

 10-15: 1, and the costa ending at about the middle of the leaf. 



