OF WESTERN PENNSYLVANIA 265 



Key to the Species. 

 a. Stem-creeping; leaves erect-spi-eading; median leaf-cells about 



2-6:1. b. 



a. Stem often ascending or erect; leaves mostly widely to squarrosely 

 spreading; median leaf-cells mostly 4-8:1, or rarely 10-15:1. 



f. 

 b. Cells in middle of leaf about 2-4:1. c. 

 b. Cells in middle of leaf about 4-6(-8):l. e. 

 c. Very slender; costa thin, ending near the middle of the leaf. 



1. A. serpens. 

 c. Less slender; costa stronger, almost reaching apex. 



d. 

 d. Stem-leaves ovate-acuminate, acute, slenderly acuminate. 



2. A. varium. 



d. Stem-leaves ovate-cordate, abruptly narrowed to a rather blunt 

 acumination. 3. A. orthodadon. 



e. Costa reaching to three-fourths the length of the leaf. 



4. A. juratckanuiu. 

 e. Costa reaching about to the middle of the leaf. 



5. A. radicate. 



f. Slender: mediati leaf-cells prosenchj'matous, hexagonal to 

 linear, 4-8(-10) :1. 6. A. kochii. 



'' f. Rather robust: median leaf-cells elongate-prosenchymatous to 

 linear, 5-10(-15):l. 7. A. riparium. 



I. Amblystegium serpens [Linnjeus] Bryologia Europsea. 



(Hypnum serpens Iwinnseus) 

 (Plate .XXXVII) 

 Dull, more or less yellowish-green, very small and slender, 

 forming thin, soft, densely interwoven mats : stems prostrate, 

 radiculose, irregularly branching, the branches ascending or 

 spreading or erect; leaves rather crowded, when moist vari- 

 ously spreading, when dry more or less appressed and im- 

 bricate; stem-leaves lance-ovate to ovate-acuminate, usually 

 long-acuminate, the largest about 0.8-1.0X0.4-0.5 mm., often 

 much smaller, narrowed and decurrent at base, slightly denticu,- 

 late or entire, somewhat concave, the margins plane ; costa 

 usually reaching about to the middle of the leaf or above, 

 often quite faint and indistinct ; branch-leaves similar but 

 smaller and narrower, usually more lanceolate ; median leaf- 

 cells oblong- to rhomboid-hexagonal, about 2-4:1, the basal 

 broader and more rectangular, the alar quadrate to transversely 

 elongate but not forming a well-defined group, some of the 

 apical considerably longer; perichsetial leaves lanceolate, thin, 

 plicate, up to 1.5 mm. long: seta rather slender, 1-3 cm. long, 

 reddish, dextrorse; capsule cylindric, the urn about 1.5 mm. 

 long, strongly curved, cernuous, constricted below the mouth 

 when dry ; lid convex-conic, rather obtusely apiculate ; peris- 

 tome rather large for the capsule, typically hypnaceous, teeth 

 pale castaneous, strongly trabeculate, below dorsally cross- 

 striolate, the dorsal lamellae projecting to form a more or less 

 crenate hyaline margin ; segments about as long as teeth. 



