Tortella undetermined

KEY TO IDENTIFICATION OF TORTELLA SPECIES IN THE RUSSIAN ARCTIC

1. Leaves fragile; leaf apices modified, propaguloid, regularly broken off ............................................. 2

— Leaves not or irregularly fragile; leaf apices not modified ....................................................................... 3

2. Stems with differentiated central strand; propaguloid leaf apices with several clear constrictions, divided into short barrel-shaped fragments; adaxial surface of costa formed of quadrate, papillose cells; basal laminal cells snow-white hyaline; border of smooth narrow cells at upper leaf margins absent .. ............................................................... T. alpicola

— Stem central strand not differentiated; propaguloid apices without or with few constrictions; adaxial surface of costa formed of elongate, smooth cells; basal laminal cells yellowish; border of 1–2 rows of smooth narrow cells at upper leaf margins often present .... ................................................................ T. fragilis View in CoL

3. Costa with quadrate, papillose cells on ventral surface from above base; central strand absent ......... 4

— Costa elongate cells on ventral surface, smooth throughout; central strand absent or present ....... 5

4. Leaves usually not undulate when wet; transition between smooth, yellowish basal cells and papillose limb cells gradual, U-shaped ............... T. splendida View in CoL

— Leaves undulate at margins when wet; transition between smooth, hyaline basal cells and papillose limb cells abrupt, V-shaped .................... T. tortuosa View in CoL

5. Leaves oblong-lanceolate, acute at apex; upper and median laminal cells with high, pedunculate papillae; stem central strand absent ................... T. densa View in CoL

— Leaves linear-lanceolate, gradually narrowed to the apex, acuminate; upper and median laminal cells with low papillae; stem central strand usually present, rarely indistinct or absent ...................................... 6

6. Smooth basal laminal cell extend upwards higher along leaf margins than near the costa, forming hyaline border; transition between smooth limb cells and papillose laminal cells +sharp, V- or U-shaped ............................................................. T. fleischeri View in CoL

— Smooth basal laminal cell extend upwards higher near the costa than along leaf margins or at equal height; transition between smooth limb cells and papillose laminal cells gradual .................................. 7

7. Stems with large central strand, 6–15 cells wide; smooth basal laminal cell extend upwards higher near the costa than along leaf margins; cells near margins in several rows hyaline, wider than juxtacostal cells, forming a distinct border; upper laminal cells 8–11(–13) µm wide ............... 12. T. spitzbergensis

— Stems with slender central strand 3–4 cells wide; smooth basal laminal cell extend upwards equally high near the costa and at leaf margins; basal marginal cells equal in width or slightly wider than juxtacostal cells, not forming a distinct border; upper laminal cells (7)11–14(16) mm wide ...................... .................................................... T. cuspidatissima View in CoL