Calliergon undetermined

KEY TO CALLIERGON SPECIES IN RUSSIA

1. Costa in most leaves to (0.4–)0.5–0.8 the leaf length ................................................... 1. C. richardsonii View in CoL

— Costa in most leaves to 0.9–1.0 the leaf length ... 2

2. Alar cells gradually grading into other laminal cells; autoicous; leaf marginal cells often form more or less distinct border ............................. 2. C. cordifolium View in CoL

— Alar cells rather abruptly delimited from other laminal cells; dioicous; leaf marginal cells never form border .................................................................... 3

3. Stem leaves (2.5–)3.0–4.0(–5.0)×1.5–2.0(–3.0) mm; alar group reaching 0.4–0.7 of distance to costa .... .............................................. 3. C. megalophyllum View in CoL

— Stem leaves (1.2–)1.5–2.7(–3.3)×0.9–1.5(–2.2) mm; alar group reaching (0.5–)0.7–1.0 of distance to costa .............................................................................. 4

4. Plants medium-sized; costa weak, (45–)60–80(–130) µm wide at base; stem leaves (1.5–)1.7–2.5(–3.3) ×(0.9–)1.2–1.4(–2.2) mm; alar cells extend to (0.5–) 0.7–0.9(–1.0) of distance to costa ........................... .......................... 4a. C. giganteum subsp. sibiricum

— Plants medium-sized to large; costa strong, being (80–)110–160(–210) µm wide at base; stem leaves (1.5–)1.7–2.7(–3.0)×(0.9–)1.2–1.5(–1.8) mm; alar cells extend to 0.8–1.0 of distance to costa ............. ........................ 4b. C. giganteum subsp. giganteum View in CoL