The genus Pohlia usually has
a well-developed, double peristome with non-appendiculate cilia. Its leaves are
lanceolate to ovate-lanceolate, have long leaf cells, unbordered margins that
are usually serrate to serrulate above, and strong, single costae that are
generally subpercurrent to percurrent and only occasionally shortly excurrent.
There are three distinct groups within Pohlia: subg. Pohlia,
subg. Nyholmiella Shaw, and subg. Cacodon Lindb. (Shaw 1984a).
Subgenus Pohlia is mostly a
Northern Hemispheric group. Plants in this subgenus generally lack axillary
gemmae, their capsules have elongate to rectangular, straight-walled exothecial
cells, superficial stomata, and compound, revoluble annuli. Their peristomes
are generally well-developed with reddish orange to brownish exostome teeth
that are bordered, strongly trabeculate, and coarsely papillose above, but
finely papillose below. The endostome basal membranes in subg. Pohlia
can be low or high, but the segments are generally broad, keeled, and variously
perforate, and the cilia vary from long and nodose to short and rudimentary.
Subgenus Nyholmiella is
mostly a Southern Hemispheric group. Plants in this subgenus often have
axillary gemmae, capsules with rectangular to isodiametric, straight-walled or
sinuose (often bulging) exothecial cells, superficial or sunken stomata, and
compound annuli. Their peristomes are usually reduced with white to whitish
yellow, narrow exostome teeth that are not bordered, weakly trabeculate, and
densely, evenly papillose throughout. The endostome basal membranes in subg. Nyholmiella
can be low or high, but the segments are linear, flat or weakly keeled, and not
or weakly perforate, and the cilia are absent or rarely rudimentary.
Subgenus Cacodon includes
those species which have in the past been treated in the segregate genus Mniobryum
B.& S. ex Limpr. Plants in this subgenus frequently have
axillary gemmae or rhizoidal tubers, capsules with short rectangular to
isodiametric, strongly sinuose exothecial cells, superficial or immersed
stomata, and poorly developed, non-revoluble annuli. Their peristomes are
well-developed with reddish orange to brownish exostome teeth that are
bordered, strongly trabeculate, and coarsely papillose above but finely
papillose below. The endostome basal membranes in this subgenus can be low or
high, but the segments are broad, keeled, and widely perforate, and the cilia
are long and nodose.
Pohlia grades into Bryum
in nearly every feature and there are some Bryum species (e.g., B.
apiculatum) that occasionally are treated in Pohlia. As a result of
this morphological intergradation Pohlia can only be distinguished from Bryum
by a suite of characters that include long, narrow, unbordered leaves, long,
narrow leaf cells, costae percurrent (or just barely excurrent), and nodose
endostomial cilia (although P. nutans can have appendiculate cilia).
Remarkably, Buck & Goffinet (2000) placed these very similar genera in
different families (Bryum, Bryaceae; Pohlia, Mniaceae). As noted
by Shaw (1994) when sporophytes are absent the genera Pseudopohlia, Mielichhoferia
and Schizymenium cannot be distinguish from Pohlia without prior
experience with the individual species.
Shaw (1982a) revised the genus Pohlia
in North and Central America and the West Indies.