Didymodon

Beever, Jessica E. & Fife, Allan J., 2016, A new species of Didymodon (Pottiaceae, Bryophyta) and a key to New Zealand representatives of the genus, Phytotaxa 258 (3), pp. 279-286 : 284

Key to New Zealand species of Didymodon and Gymnostomum

As is the case in North America ( Zander 1998), some species of the genus Didymodon in New Zealand are common and often difficult to distinguish. The colour reaction of the walls of upper laminal cells with 2% KOH is a useful identification character.As noted by Zander (1993, p. 3), to assess the colour reaction accurately it is necessary to focus on cell walls under high power of the compound microscope.

1. Plants forming dense, bright yellow-green turves, on calcareous substrates; mid-laminal cells papillose, with 4−6 conspicuous papillae per cell; leaf margins plane; leaf apex flat, unistratose, acute to rounded-obtuse and often variable on the same plant; costa failing below the leaf apex ....................................................................................................................... Gymnostomum calcareum

- Plants forming dense turves or loosely to densely gregarious, of various dark colours and not bright yellow-green, on various mineral substrates including calcareous; mid-laminal cells papillose, with 1−3(–4) low, often inconspicuous, papillae, or papillae lacking; leaf margins recurved at least in part, or rarely plane (if plane, plants brownish green); leaf apex flat or cucullate, unistratose or otherwise, acute to rounded-obtuse (but not on the same plant); costa failing below the leaf apex, or percurrent to excurrent .... ....................................................................................................................................................................................... 2 Didymodon

2. Leaves lingulate, scarcely tapered from base to apex; lamina unistratose throughout; stem in cross-section without a central strand ................................................................................................................................................................................................ 3

- Leaves lanceolate or lingulate from an ovate base, widest near the leaf base; lamina unistratose throughout or bistratose at distal margins; stem in cross-section with a well-developed central strand .............................................................................................. 4

3. Leaf margins plane or weakly recurved in mid-leaf; KOH reaction of walls of upper laminal cells yellow; setae 3–6 mm; operculum mammillate with a blunt central projection; peristome absent ..................................................... Didymodon novae-zelandiae

- Leaf margins recurved, at least below; KOH reaction of walls of upper laminal cells orange; setae 6–12 mm; operculum rostrate with a long oblique beak; peristome present ................................................................................................ Didymodon weymouthii

4. Laminal cells smooth; perichaetial leaves strongly differentiated, with sheathing base narrowing abruptly to a subula; species known only from Hawke’s Bay and Wellington Land Districts, not seen since 1956 ........ Didymodon calycinus Dixon (1915: 95)

- Laminal cells papillose, with low, rounded, simple papillae; perichaetial leaves weakly differentiated, base may be sheathing but leaves gradually narrowing to their apex, not subulate; common species, widespread in New Zealand......................................... 5

5. Leaves lingulate from an ovate base; leaf apex obtuse-rounded; costa clearly failing ca. 4−8 cells before unistratose leaf apex ..... ......................................................................................................................................................................... Didymodon tophaceus

- Leaves linear-lanceolate to broadly lanceolate; apex acuminate to bluntly acute; costa percurrent, excurrent, or terminating obscurely in pluristratose leaf apex ...................................................................................................................................................... 6

6. Leaves linear-lanceolate; basal laminal cells strongly differentiated, elongate, thin-walled and hyaline in ca. 10 rows (discernible as a white leaf base with 10× hand-lens); stem in cross-section with cells of outer layer not strongly differentiated, thin-walled (at least on outermost wall); peristome not spirally twisted, teeth erect and incurved when dry ............................................................. ............................................................................. Didymodon australasiae ( Hooker & Greville 1824: 301) R.H. Zander (1978: 21)

- Leaves broadly lanceolate; basal laminal cells not strongly differentiated except in 1 or 2 rows at extreme base, not hyaline (not discernible with 10× hand-lens); stem in cross-section with a differentiated outer layer of thick-walled, narrow-lumened stereid cells; peristome spirally twisted ...................................................................................................................... Didymodon torquatus