Monographs Details: Fissidens petrophilus Sull.
Description:Species Description - Plants bright green. Stem monomorphic, to 20 mm [2 cm] long x 2.5 mm wide, unbranched and branched, ± inrolled from tips when dry; rhizoids basal, smooth, reddish-brown; axillary hyaline nodules absent; epidermis and outer 1-2 tiers of cortical cells small, incrassate, usually pigmented; inner cortical cells larger, thin-walled, hyaline; central strand present. Leaves little changed when dry, tightly imbricate, as many as 38 pairs, narrowly lanceolate, acute, to 3.3 mm long x 0.3-0.4 mm wide, perichaetial leaves longest; margin entire, elimbate; costa ending 6-15 cells below apex, oblongifolius type, distal part in transverse section showing 2 stereid bands separated by 4 enlarged cells arranged in 2 rows; dorsal lamina narrowed to insertion, reaching insertion or ending above; vaginant laminae of cauline leaves 1/2-3/4 leaf length, unequal, minor lamina of cauline leaves narrowed and ending on costa or rounded and free or acute and ending ± 1/2 distance between costa and margin; laminal cells distinct, eguttulate, unistratose, firm-walled, smooth, plane throughout, 8.5-13 µm long, marginal cells smaller, 7-9 µm long, juxtacostal cells in lower part of vaginant laminae larger, quadrate to oblong, to 33 |xm long. Monoicous (gonioautoicous); perigonia gemmiform, axillary; perichaetial terminal. Sporophytes 1 per perichaetium, yellow, darkening with age; seta smooth, to 3 mm long; theca exserted, erect, radially symmetric, 0.9 mm long, stomatose, exothecial cells quadrate to oblong, collenchymatous; peristome similiretis type; operculum conic, long-rostrate, 0.5 mm long. Spores smooth, 11-14 µm diam. Calyptra not seen.
Discussion:Fissidens petrophilus is distinguished by vaginant laminae that are 1/2-3/4 the leaf length, a minor lamina that is either narrowed distally and ending on or near the costa or rounded and free, or acute and ending about 1/2 the distance between costa and margin, a costa that ends 6-15 cells below the leaf apex, smooth, plane laminal cells throughout, and erect, radially symmetric thecae. It is closest to F. imbricatus which has vaginant laminae that are 4/5 or more the leaf length, a minor lamina that usually ends between the costa and margin, a costa that ends 3-5 cells below the leaf apex, and lenticularly thickened cells in the dorsal and ventral laminae. Both species show four enlarged cells in 2 rows in transverse section of the distal part of the costa.
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