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Warnstorfia exannulata
Distinguished from W. fluitans by having a stronger costa, reddish coloration, and abruptly inflated groups of hyaline alar cells. In general, the plants are larger and more robust then those of W. fluitans. The denticulate leaf margins, often seen best in the apical tufts of young leaves, distinguish this species and W. fluitans from species in the Drepanocladus aduncus group. Scorpidium species have shorter leaves and enlarged epidermal cells of the stem, while Hamatocaulis species have no central strand of the stem, and lack differentiated alar cells. Somewhat similar species include W. trichophylla with a long excurrent costa and Calliergidium (Drepanocladus) tundrae with 5-angled stem and long decurrent leaves.
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| Habit | Slender to large, reddish-green plants. |
| Stems | sporadically to irregularly pinnately branched, in transverse section with central strand and no enlarged epidermal cells. |
| Leaves | narrowly lanceolate, gradually acuminate, with denticulate margins, sometimes with teeth only near apex and sometimes teeth continuing to mid leaf and conspicuous on the lower shoulders, costa strong, ending 2/3 up the leaf to just below apex, excurrent in closely related aquatic species. |
| Leaf Cells | elongate-linear with rather blunt ends, smooth. |
| Alar Cells | abruptly inflated and hyaline, forming conspicuous groups of one row of enlarged, rectangular cells along with a few smaller, hyaline cells. |
| Habitat | Floating in water or forming emergent mats in pools or on unconsolidated peat. Characteristic of poor fens. |
| Common Associates | In boreal regions found with Sphagnum majus, S. fallax, S. obtusum, and S. jensenii, occasionally with the rare S. lindbergii and farther north with S balticum. |
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