Polytrichum Hedwig, 1801.

On a worldwide scale, most members of the genus Polytrichum and its mostly tropical relative Pogonatum have parallel photosynthetic lamellae that run from near the base to near the tip of the adaxial surface of the leaf. When viewed in cross-section, the terminal cells of the lamellae (marginal if viewed in a lamellar scrape) are somewhat larger in diameter and of different morphology from the more proximal cells of that lamella.

Key to Polytrichum

The plants that we include in this key are robust plants whose generally stiff leaves. In all the plants here the photosynthetic lamellae are so numerous as to cover most of the leaf width. In our Polytrichum, these terminal cells are smooth, whereas in plants treated under the Key to Pogonatum Etc. the terminal cells are variously papillose.

Species included are all in Polytrichaceae:
Polytrichum commune Hedwig
Polytrichum formosum Hedwig
Polytrichum juniperinum Hedwig
Polytrichum longisetum Swartz ex Bridel
Polytrichum piliferum Hedwig
Polytrichum sexangulare Flörke ex Bridel
Polytrichum strictum Bridel

Identification of Polytrichum or its relatives involves making a leaf cross-section so that the distal cell of the photosynthetic lamellae can be viewed. Alternatively or additionally the lamellae can be scraped from an individual leaf with the broad side of a razor blade. If mistakenly keyed here, species included in the Key to Pogonatum Etc. will end up at couplet F.

A. Leaf margins entire, erect to incurved at margin .....B
A. Leaf margins usually dentate or, if entire, without the strong incurving of the margin .....E

B. Leaves strongly cucullate at apex; plant small with leaves mostly less than 5 mm long .....Polytrichum sexangulare
B. Leaves mostly flat at apex, denticulate to dentate at least at apex .....C

C. Leaf with a strongly hyaline apex that is reddish only in a small portion of its base, if at all .....Polytrichum piliferum
C. Leaf with a reddish to concolorous apex, hyaline only at extreme tip, if at all .....D

D. Plants mostly more than 10 cm high, occupying bogs and very wet sites, often with Sphagnum; leaves rigidly erect when dry; stems densely covered with white to pale brown rhizoids .....Polytrichum strictum
D. Plants mostly shorter, occupying mesic to rather dry soil, never in bogs; leaves ascending when dry; stems with only sparse rhizoidal cover .....Polytrichum juniperinum

E. Marginal cells of the lamellae notched in cross-sectional view, without either papillae or cuticular ridges .....Polytrichum commune
E. Marginal cells otherwise .....F

F. Laminar cells 15-25 µm in longest diameter; mid-leaf with lamina 6 or more cells wide; capsule inconspicuously sulcate .....Polytrichum longisetum
F. Laminar cells 10-15 µm in longest diameter; mid-leaf with lamina 1-5 cells wide; capsule strongly sulcate .....Polytrichum formosum