Gymnostomum Nees & Hornschuch in Nees, Hornschuch & Sturm, 1823.

Gymnostomum calcareum, and Ron Coleman


Key to Gymnostomum Etc.

In this key we treat acrocarpous, costate and pluripapillose plants mostly with entire and plane margins. These plants are usually calciphiles or on desert seepages, especially along crevices of shaded rock underhangs. One of the most unusual features of these plants is the great variability of size and shape of the basically isodiametric cells.

Species included in this key are in Pottiaceae, except Zygodon (Orthotrichaceae):
Anoectangium aestivum (Hedwig) Mitten, not known from CA
Eucladium verticillatum (Hedwig in Bridel) Bruch & W. P. Schimper
Gymnostomum aeruginosum J. E. Smith
Gymnostomum calcareum Nees & Hornschuch
Gymnostomum viridulum Brid., recognized from CA since 2004
Hymenostylium recurvirostrum (Hedwig) Dixon
Molendoa sendtneriana (Bruch & W. P. Schimper) Limpricht
Zygodon rupestris W. P. Schimper ex Lorentz

Gymnostomum is restricted to calcareous, usually limestone, habitats throughout the state. While it can form small tufts of only a few loosely placed plants, it almost always forms large and very tight, somewhat glaucous, cushions. In areas of long-duration seeps over limestone, it may form cushions several square meters in size. The larger cushions of Gymnostomum when vertically split often reveal internal layering that may represent annual increments of growth. This plant can be confidently identified when one sees the rather short leaves with plane margins and papillose leaf cells. Anoectangium has these same features but it is not yet found in the state. A field character for this latter genus is the hint of tristichous leaf arrangement, a feature not present in Gymnostomum. Another useful field character for Gymnostomum is the tendency for colonies to dry and crack into rectangular blocks like dried clay. There is great diversity of opinion on whether to recognise the two Western North American species of Gymnostomum. After much hesitation, we suggest that all the Californian material that we have examined falls into G. calcareum, and we find the nearest localities for G. aeruginosum in Washington State. We have become convinced of their distinction not only by the cell size differences and by the larger plant typical of G. aeruginosum but, more importantly, by the looser clones of that latter species.

A. Leaves terminating in a single elongate and more or less hyaline cell that differs strongly from the isodiametric subapical cells; cells of leaf lamina with multiple, sharp and discrete papillae; axils of younger leaves usually with stalked, elliptical, septate gemmae .....Zygodon: Z. rupestris
A. Leaf apices acute but mostly without a differentiated apical cell; cells of leaf lamina with multiple, blunt papillae that seem to merge with neighboring papillae in a complex of roughenings; gemmae scarce, not stalked .....B

B. Leaf margin at junction between the basal and upper laminal cells with several teeth .....Eucladium: E verticillatum
B. Leaf margin entire throughout .....C

C. Margin recurved at extreme basal margin on at least one side of some of the leaves; extreme distal cell of leaf pellucid and somewhat enlarged .....Hymenostylium: H recurvirostrum
C. Margins plane throughout; distal cell of leaf not so differentiated .....D

D. Costa on adaxial surface with quadrate cells little different from adjacent laminal cells; leaves not adaxially grooved; leaves spirally arranged along stem .....E
D. Costa with elongate cells on adaxial surface; leaf deeply grooved by ascending lamina on each side of costa on at least the distal one-half; perichaetia lateral; leaves obscurely tristichous .....F

E. Costa less than 50 µm wide at base; cells near leaf apex 7-8 µm wide .....Gymnostomum calcareum
E. Costa more than 60 µm wide at base; cells near leaf apex more than 10 µm wide .....Gymnostomum aeruginosum

F. Leaves concave, especially near apex; costa of well developed leaves with two stereid bands .....Molendoa: M. sendtneriana
F. Leaves keeled along costa; costa with only one stereid band .....Anoectangium: A. aestivum not known from CA