Didymodon, S. STR.

Ignatova, E. A., Fedosov, V. E., Kuznetsova, O. I., Fedorova, A. V. & Ignatov, M. S., 2024, On the genus Didymodon s. str. (Pottiaceae, Bryophyta) in Russia, Arctoa 33 (2), pp. 129-155 : 152-153

KEY TO IDENTIFICATION OF DIDYMODON S. STR. IN RUSSIA

1. Leaves fragile; leaf apices caducous, differentiated into swollen, rhomboidal propagula consisting entirely of the costa ................... D. anserinocapitatus

— Leaves not fragile or, rarely, with leaf tips broken off but not differentiated into propagulae ................... 2

2. Plants bluish green; leaves linear-lanceolate, usually crisped when dry; basal laminal cells strongly differentiated, hyaline, thin-walled ........... D. glaucus

— Plants green, yellowish-green or brown; leaves ovate, ovate-lanceolate, or lineaer-lanceolate, not crisped when dry; basal cells not or weakly differentiated, firm-walled ............................................................ 3

3. Leaf margins crenulate by bulging cell walls ...... 4

— Leaf margin smooth .............................................. 6

4. Upper leaf cells covered with low and flattened papillae visible only in leaf transverse section; gemmae absent ....................................... D. nigrescens

— Leaf cells smooth; unicellular gemmae occasionally present .................................................................... 5

5. Plants with fragile flagelliform innovations; leaves dimorphic ............................... D. subandreaeoides

— Plants without fragile flagelliform innovations; leaves monomorphic .................................... D. perobtusus

6. Ventral (adaxial) surface cells of costa in the upper half of leaf elongated, not transient from lamina 7

— Ventral (adaxial) surface cells of costa in the upper half of leaf ± isodiametric, transient from lamina 8

7. Leaves ovate-lanceolate, acuminate, catenulate when dry; multicellular gemmae usually present in leaf axils ............................................. D. maschalogena

— Leaves ovate, truncate at apices, not catenulate when dry; gemmae absent .............. D. truncatus (p. 152)

8. Upper leaf margins regularly bistratose in 1–3 cell rows ....................................................................... 9

— Upper leaf margins unistratose ......................... 10

9. Leaf apices not sharp; costa not sharply delimited from the lamina in distal part of leaf acumen, percurrent .................................................. D. rigidulus

— Leaf apices sharp; costa sharply delimited from the lamina, long excurrent ............ D. daqingii (p. 148)

10. Laminal cells smooth ......................................... 11

— Upper and median laminal cells papillose on both surfaces or only on dorsal side .......................... 15

11. Costae long excurrent; leaf tips fragile, often broken off ....................................... D. abramovae (p. 144)

— Costa percurrent or short excurrent; leaf tips not fragile ....................................................................... 12

12. Leaves 1.6–3.2 mm long, with acumina longer than bases .......................................... D. validus (p. 140)

— Leaves 0.4–1.8 mm long, with acumina equal in length to bases or slighly longer ....................... 13

13. Leaves 0.8–1.8× 0.3–0.65 mm; laminal cells thick-walled, with rounded lumens ................... D. acutus

— Leaves 0.5–1.3× 0.25–0.5 mm; laminal cells thin-walled or with moderately thickened walls, with subquadrate, rectangular or ovate lumens ............... 14

14. Leaves 0.6–0.85× 0.25–0.3 mm; in mesic habitats. ................................................. D. borealis (p. 141)

— Leaves 0.5–1.3× 0.4–0.5 mm; in xeric habitats ....... .................................... D. mongolicus p.p. (p. 148)

15(10). Upper and median laminal cells equally papillose on both surfaces ........................................... 16

— Upper and median laminal cells papillose only or predominantly on dorsal surface, occasionally with few papillae on ventral side ............................... 22

16. Leaf margins recurved at lower 1/2–3/4, rarely almost to the apex .................................................. 17

— Leaf margins revolute almost to apex ................ 20

17. Upper leaves erectopatent when wet, with weakly differentiated bases, gradually narrowed into acumina; upper and median laminal cells densely papillose, papillae simple and bifid ... D. calciphilus (p. 151)

— Leaves widely spreading to squarrose when wet, gradually or abruptly narrowed into long acumina (long- er than bases); upper and median laminal cells with sparse, simple papillae ....................................... 18

18. Leaves with narrow bases, 0.3–0.35 mm wide, from short-rectangular bases gradually narrowed into acumina, to 1.9 mm long .......... D. cherdantsevae (p. 145)

— Leaves with wider bases, 0.6–0.9 mm wide, from ovate bases +abruptly narrowed into acumina, to 3.2 mm long .............................................................. 19

19. Leaves 1.8–2.7× 0.6–0.9 mm; costa percurrent; leaf margins recurved to 3/4 of leaf length; gemmae absent ............................... D. hengduanensis (p. 147)

— Leaves 1.5–3.2× 0.5–0.8 mm; costa excurrent; leaf margins recurved almost to the apex; ovate multicellular gemmae often present in leaf axils ............ ..................................................... [ D. schensianus ]

20(16). Costa in transverse section with guide cells in 2- 3 layers, without ventral stereids ............................. ............................................ D. baicalensis (p. 149)

— Costa in transverse section with guide cells in 1 lay- er, ventral stereid band present ........................... 21

21. Leaves with cordate bases ................... D. cordatus

— Leaves with square bases .................. [ D. tectorum ]

22(15). Plants tiny, leaves to 0.45 mm long, with flat margins ................................................ D. tibeticus

— Plants larger, leaves longer than 0.45 mm, with narrowly recurved margins ...................................... 23

23. Costa long-excurrent ..... D. icmadophilus (p. 142)

— Costa percurrent to short-excurrent ........................ .................................... D. mongolicus p.p. (p. 148)