20. Rhexophyllum Herzog, Biblioth. Bot. 87: 38. 1916.
[Greek rhexis, breaking, and phyllon, leaf, alluding to fissile distal leaf margins]
Patricia M. Eckel
Neocardotia Thériot & E. B. Bartram
Plants turf-forming, dull green to blackish green distally, reddish brown proximally. Stems elongate, 2-4 cm; hyalodermis present, sclerodermis strong, central strand present; axillary hairs of ca. 10 cells, all hyaline, or proximal 1-3 cells yellow-brown. Leaves appressed-incurved and loosely twisted-flexuose when dry, limb widely spreading to squarrose beyond an expanded base when moist; lanceolate, deeply and narrowly keeled distally; base rectangular, somewhat clasping, shortly decurrent; leaf margins recurved in proximal 1/4-1/2, coarsely, irregularly and sharply papillose-dentate to erose-dentate in distal 1/4-1/2, teeth multicellular, fragile margins tending to split in irregular horizontal lines, lamina 2-stratose in patches, sometimes only on the leaf margins; apex narrowly to broadly acute; costa excurrent as a sharp, denticulate mucro, adaxial outgrowths absent, adaxial cells elongate, short-rectangular to quadrate distally, 3-4 across costa at mid leaf; transverse section reniform, adaxial epidermis weakly differentiated or absent, two stereid bands present, guide cells 2-4 in 1 layer, hydroid strand variably present, abaxial stereid band reniform-semicircular in shape, abaxial epidermis strongly differentiated; basal cells nearly undifferentiated on the margins, gradually differentiated in the middle of the leaf base, oblong-rectangular, somewhat wider than distal cells, 3-5:1, walls thin; cells of limb subquadrate to rounded-hexagonal, 1:1, 2-stratose in patches, sometimes only on the leaf margins, papillae solid or hollow, crowded, multifid, walls thin, convex on both surfaces. Specialized asexual reproduction lacking. [Sexual condition dioicous; perichaetia terminal, interior leaves highly differentiated, long-lanceolate, convolute-sheathing. Seta to 1 cm. Capsule stegocarpous, theca ellipsoidal to cylindric; annulus of 2 rows of strongly vesiculose cells; operculum short-conic to conic-rostrate, somewhat inclined; peristome absent. Calyptra cucullate. Spores 11-18 µm.] Laminal KOH color reaction red.
Species 1: sw United States, Mexico, South America.
The significant character separating Rhexophyllum from the similar genus Leptodontium is the presence of a stem central strand. There are apparently no close relatives although R. H. Zander’s (1993) phylogenetic study indicates that the genus shares an ancestor with Mironia, a similar Latin-American genus with species whose leaves are 2-stratose only along the margins of the distal lamina, and whose capsule is peristomate.
Zander, R. H. 1994o. Rhexophyllum. In: A. J. Sharp et al., eds. The moss flora of Mexico. Mem. New York Bot. Gard. 69: 260.