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 sarmentosum is distinguished by the oblong or ovate stem leaves suddenly narrowed to rounded- or acute-apiculate apices. The apiculus is most distinct in young leaves and is rarely completely absent, but may be eroded in older leaves. The species could be confused with species of Calliergon, which are larger, have broader leaves, and never have a clear red color, or with Straminergon stramineum, which is never red and lacks an apiculus. Straminergon stramineum is more sparsely branched (sometimes unbranched) than Sarmentypnum sarmentosum, and the alar regions are ovate or broadly ovate in Straminergon, transversely triangular in S. sarmentosum.

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