Monographs Details: Porotrichum
Authority: Buck, William R. 1998. Pleurocarpous mosses of the West Indies. Mem. New York Bot. Gard. 82: 1-400.
Description:Genus Description - Plants medium-sized to robust, rarely small unless juvenile, stipitate-frondose, in pale- to dark-green to golden, mostly stiff, often extensive colonies. Primary stems creeping, secondary stems mostly arising from upturning primary stem, the creeping stem continuing by innovations near the base of the stipe, the secondary stems distinctly stipitate, erect to pendent, frondose, regularly to irregularly 1-2(-3)-pinnate, usually complanate-foliate, the primary stem often more slender than the stipe, the ultimate branches often attenuate, often with flagellate branches; in cross-section with small thick-walled cells surrounding larger thinner-walled cells, central strand almost always present; paraphyllia absent; pseudoparaphyllia narrowly to broadly foliose; axillary hairs with 1-2 short to rectangular brown basal cells and 1-3 elongate hyaline to brown distal cells. Primary creeping stem leaves reduced, widely spaced, appressed or with spreading apices, ovate to triangular, acuminate, not decurrent; margins subentire, usually plane, sometimes incurved; costa single, ending near or somewhat above midleaf; cells ± linear, smooth, firm- to thick-walled, ± porose; alar cells not or weakly differentiated. Stipe leaves often widely spaced, sometimes imbricate, appressed to squarrose, not complanate, ovate to triangular, acuminate or less often obtuse or acute, usually not or scarcely decurrent; margins subentire, rarely toothed, plane to incurved above or recurved below; costa single, ending near or above midleaf; cells mostly long-fusiform, smooth or rarely prorulose, firm- to thick-walled, ± porose; alar cells mostly weakly differentiated. Secondary stem (continuation of stipe above branching) leaves differentiated from those of the stipe and usually from branch leaves, typically the largest leaves on the plants, complanate, erect-spreading to spreading, usually oblong-ovate, obtuse-cuspidate to acute or short-acuminate, rarely plicate when dry, not or scarcely decurrent; margins mostly serrate at least above, less often throughout, the teeth 1-3-celled, plane to incurved above or recurved below; costa single, usually ending 2/3-3/4 the leaf length, projecting as a spine at apex or not; cells mostly fusiform, ca. 6-8:1, sometimes shorter or longer, smooth or prorulose, firm- to thick-walled, rarely thin-walled, often porose, often becoming shorter apically; alar cells typically not or poorly differentiated. Branch leaves erect- to wide-spreading, complanate or less often ± turgid, ovate to oblong-ligulate or rarely ± lingulate-obovate, often concave, sometimes strongly so, sometimes plicate, especially when dry, not or scarcely decurrent, not rounded to the insertion; margins typically serrate above with teeth 1-3-celled, sometimes almost to base, rarely with just a few scattered teeth, plane to incurved above, less often recurved below; costa single, ending near midleaf to ca. 5/6 the leaf length, rarely shorter, projecting as a spine at apex or not; cells mostly fusiform, ca. 6-8:1, sometimes shorter (1-2:1) to linear, smooth or prorulose, often firm- to thick-walled and ± porose, often becoming shorter in the apex; alar cells not or weakly differentiated or distinct but few. Asexual propagula common, of flagellate branches or rarely of caducous leaves; flagellate branches arising from branch apices or axils of branch leaves, with greatly reduced leaves. Dioicous or rarely autoicous. Perichaetial leaves typically oblong-lanceolate, with spreading apices, gradually or abruptly acuminate; margins subentire, mostly plane; costa often weak, absent or single; cells mostly linear, smooth, thick-walled, porose. Setae short to elongate, mostly 0.5-3 cm long, usually smooth, rarely roughened above, stout to slender, reddish; capsules erect, cylindric, symmetric; exothecial cells subquadrate to short-rectangular, mostly firm-walled, becoming somewhat shorter toward the mouth but not strikingly so; annulus differentiated; operculum obliquely rostrate from a conic base; peristome double, attached at or inserted slightly below the mouth, exostome teeth on the front surface cross-striolate either only in lowermost part or well up, sometimes with overlying papillae, papillose above, scarcely to strongly trabeculate at back; endostome with a medium-high to high, mostly ± smooth basal membrane, segments mostly papillose, keeled, perforate to gaping, ca. as long as the teeth, cilia in groups of 1-3, papillose, shorter than the segments, or rudimentary to absent. Spores spherical, ± smooth to papillose. Calyptrae cucullate, naked or sparsely hairy, usually smooth or slightly roughened at the extreme apex.
Discussion:Porotrichum (Brid.) Hampe, Linnaea 32: 154. 1863; Climacium subgen. Porotrichum Brid., Bryol. Univ. 2; 275. 1827; Thamnium subgen. Porotrichum (Brid.) Kindb., Hedwigia 41: 210. 1902.
Hypnum sect. Dendroglossophyllum Müll. Hal., Syn. Musc. Frond. 2: 223. 1851. Lectotype, designated here, Hypnum porotrichum Müll. Hal. = Porotrichum longirostre (Hook.) Mitt., non Hypnum longirostre Ehrh. ex Brid., Muscol. Recent. 2(2): 154. 1801, nom. illeg.
Thamnium subgen. Camptolepis Kindb. sect. Trachyphylla Kindb., Hedwigia 41: 212. 1902.
Porothamnium M. Fleisch., Musci Buitenzorg 3: 925. 1908.
Discussion. Porotrichum is a genus of perhaps 20-30 species, best developed in tropical America but with good representation in Africa and less in tropical Asia. It is characterized by strongly stipitate-frondose plants, the leaves (at least those of secondary stems above branching) are complanate, and the setae are elongate. It dillers from some of the other allied genera by mostly fusiform laminai cells that are usually firm- to thick-walled and often porose. The alar cells are weakly or not differentiated. In our flora it differs from the other stipitate-frondose genera as follows: Pinnatella has more or less isodiametric laminal cells and exostome teeth papillose throughout; Thamnobryum has a broad costa extending almost to the leaf apex, with short apical laminal cells in a large area; Homaliodendron lacks a central strand in the stem and has very short setae (2-4 mm long).
In this treatment I have agreed with Sastre-De Jesús (1987) in synonymizing Porothamnium with Porotrichum. Traditionally (Brotherus, 1925) they have been segregated by Porothamnium having an exostome cross-striolate well up the teeth, a high basal membrane, and well-developed cilia, and Porotrichum having its exostome cross-striolate only at the base of the teeth, a lower (but still prominent) basal membrane, and no cilia. De Sloover (1983) additionally suggested that Porothamnium could be recognized by a glossy (rather than dull) seta and plants of a darker, more metallic green. From my experience, species with better developed peristomes (i.e., Porothamnium) are mostly terrestrial or epilithic, and those with more reduced peristomes (Porotrichum) are epiphytic. Of course, even within a single species one finds habitat variation, but I think that peristome reduction has been habitat-driven in the group and that species of Porothamnium, as defined peristomially, are no closer to one another than they are to species within Porotrichum. Therefore, I prefer to recognize a more broadly encompassing generic concept.
Allen (1994) provided a key to the species of Porotrichum in Central America.