P. demissum
This
arctic-alpine species is rare in Russia and occurs mostly in the
relatively dry tundra in Siberia, both in Arctic and high mountains. It is similar to other Plagiobryum
species in having curved capsules with strongly elongate apophyses,
but its stem and innovation leaves are narrowly ovate-triangular
with recurved margins rather than broadly ovate to round with
usually plane margins.
— Plants
green, pale-green, brownish, whitish or silvery, at times with a
pearl luster or green-rose sheen due to large-sized leaf cells;
innovations often julaceous, usually forming main part of tuft;
innovation leaves imbricate, broadly ovate to round, margins usually
plane, rarely recurved 2
2. Ñapsules
straight with transverse mouths; exostome about half endostome
length 4. P. japonicum
This
species is found in Far Eastern Russia. It is somewhat similar to P. zieri in having long
julaceous innovations. But differs fromB. zieri in having
more distinctly reflexed innovation leaf apices and strongly
excurrent costae that give the innovations a more“setaceous”
aspect that is especially pronounced in plants from drier habitats.However, the exostome
of P. japonicum
like that of P. demissum is shorter
than the endostome, while the exostome in P. zieri is equal the
endostome. Plagiobryum
demissum is also
similar to P. japonicum in having
spores that remain as tetrads, but its non-julaceous innovations,
narrowly ovate-triangular leaves with recurved margins and strongly
curved capsules makes it difficult to confused them.
— Capsules
somewhat curved with oblique mouths; exostome as long as endostome
3
3. Plants
pale-green to silvery, with a strong pearl luster or green-rose
sheen; julaceous innovation leaves ovate-rounded, strongly concave,
broadly tapered above, margins plane; fertile stem leaves
ovate-elliptic, moderately concave, gradually tapered above, margins
plane; capsule necks longer than urns 3. P. zieri
This
widespread Holarctic species is moderately rare on wet rocks (often
close to waterfalls) in the mountains throughout Russia, especially
in areas with calcareous bedrock. Its tumid plants are similar to
those of Bryum
argenteum in aspect,
but differ in having a curious pearl luster or green-rose sheen that
make it immediately recognizable. Plants of P. japonicum have a
similar color, but that species differs from P. zieri in having
shorter exostome teeth as well as other features noted in the key.
—Plants
pale-green to brownish, dull; innovation and fertile shoot leaves
more or less similar, ovate-elliptic, moderately concave, gradually
tapered above, margins moderately recurved; capsule necks equal to
or shorter than urns 2. P. hultenii
Plagiobryum
hultenii is known
from the Russian Far East and Japan. It differs from all other
species in the genus in having only pale-green or brownish plants. It is similar to P. demissum in having
recurved leaf margins and no conspicuous stem/branch innovations. But, differs in having larger leaves (1.3–3.00.4–1.1
vs. 0.8–1.50.2–0.4(–0.5)
mm) that are ovate-elliptic rather than ovate-triangular.