Flora of North America species comparison

Sarmentypnum exannulatum is one of the most frequent wetland species, with leaves that narrow gradually to the apex and are gradually curved throughout. The marginal laminal cells in the widest part of the stem leaves are often distinctly rectangular, distinctly widened, or both; this feature is never as pronounced in other species of Sarmentypnum as in S. exannulatum. However, because this feature is rather variable it should be used with caution when identifying material. Warnstorfia fluitans and W. pseudostraminea differ from S. exannulatum in being autoicous, having mainly narrowly triangular to lanceolate pseudoparaphyllia, hardly ever with pure red pigments, and with more weakly differentiated alar regions. The alar regions in W. fluitans usually reach less far distally in the leaf than in S. exannulatum, whereas those in W. pseudostraminea often form ovate regions together with the supra-alar cells. Phenotypes of S. exannulatum, found in springs, with inflated alar cells in more or less one row along the leaf base have been called W. exannulata var. purpurascens. However, transitions between these phenotypes and those from other habitats exist, and if the different phenotypes are cultivated together, the resulting plants cannot be separated (based on north European material).

(Discussion copyrighted by Flora of North America; reprinted with permission.)

Sarmentypnum pseudosarmentosum is superficially similar to S. sarmentosum in habit, with rather thick shoots and short and often curved branches with obtuse apices due to densely inserted and relatively short-acuminate leaves. However, the leaves are more gradually narrowed than in S. sarmentosum, and the usually acuminate or short-acuminate leaf apices in S. pseudosarmentosum make the separation easy. Sarmentypnum pseudosarmentosum is similar to S. exannulatum, from which it differs mainly in the more strongly concave leaves that are mostly deeply furrowed to almost tubular distally, and in its entire leaf margins. Sometimes the stem leaf lamina is weakly plicate (when moist) in S. pseudosarmentosum, which does not occur in S. exannulatum. Few specimens of S. pseudosarmentosum were available for study; it may be a good species or may represent an extreme, Arctic modification of S. exannulatum.

(Discussion copyrighted by Flora of North America; reprinted with permission.)