Monographs Details: Pilotrichum
Authority: Buck, William R. 1998. Pleurocarpous mosses of the West Indies. Mem. New York Bot. Gard. 82: 1-400.
Description:Genus Description - Plants small and slender to robust, in dull or lustrous, green to golden to bronzish, often small patches. Stems creeping, leaves often eroded or naked or with small scale-like leaves, the primary branches erect to pendent, pinnate, irregularly pinnate to regularly bi- to tripinnate, not complanate-foliate; in cross-section without a unistratose hyalodermis, the outer layer of small-lumened cells surrounding somewhat larger thin- to firm-walled, or rarely thick-walled cells, central strand none; rhizoids restricted to stems and bases of primary branches, smooth; pseudoparaphyllia filamentous to foliose; axillary hairs 2-celled, with a short brown basal cell and an elongate hyaline distal cell. Primary branch leaves fairly crowded, scarcely or distinctly contorted when dry, squarrose to erect, mostly erect-spreading to spreading when moist, lanceolate to ovate, acute to acuminate, sometimes asymmetric, sometimes plicate, seldom undulate, often concave; margins not bordered, subentire to serrulate, plane or incurved, sometimes narrowly recurved; costa double, usually ending 3/4 the leaf length to subpercurrent, often crested; cells mostly short-rectangular, mostly prorulose, usually firm- to thick-walled, usually porose, becoming oblong toward the insertion; alar cells not differentiated; secondary (and tertiary) branch leaves squarrose to erect, sometimes ± contorted when dry, broadly ovate to lanceolate, acute, apiculate or acuminate, often concave, not or slightly decurrent, with margins serrulate to crenulate to subentire, plane to incurved, with costa double, ending ca. 1/2 the leaf length to subpercurrent, diverging throughout or parallel above, tapering toward apex or not, often crested, with cells ± isodiametric to oblong, usually prorulose at upper ends, often thick-walled, sometimes porose, becoming longer toward insertion, usually more thick-walled and porose, without alar cell differentiation. Asexual propagula often borne on costae or leaf bases, uniseriate, usually brown, slightly roughened. Dioicous or rarely synoicous. Perichaetia ± conspicuous, on primary branches or secondary branch bases; leaves usually erect, sometimes with ± spreading apices, mostly lanceolate to oblong, acute to apiculate to acuminate; margins serrulate to subentire, plane or rarely incurved; costa double, divergent or parallel, rarely absent, not or rarely crested; cells mostly oblong, prorulose or smooth, thick-walled, ± porose, becoming longer and colored toward insertion. Setae short, rarely exceeding 2 mm, often stout, smooth; capsules exserted or rarely immersed, erect, ± ovoid to cylindric, symmetric; exothecial cells short-rectangular, firm-walled, not collenchymatous; annulus differentiated; operculum conic-rostrate; columella not exserted, usually with an enlarged, knob-like apex; peristome double, attached near the mouth, exostome teeth pale, narrowly triangular, not or narrowly bordered, not shouldered, on the front surface with a straight or obscurely zig-zag median line, spiculose throughout, usually somewhat trabeculate at back; endostome pale, spiculose, with a low basal membrane, segments narrow, keeled, not or narrowly perforate, often with baffle-like crosswalls, ca. as long as or shorter than the teeth, cilia mostly none, rarely rudimentary. Spores spherical to oval, almost smooth to finely papillose. Calyptrae mitrate, lobed at base, naked or hairy, smooth.
Discussion:Pilotrichum P. Beauv., Mag. Encycl. 5: 327. 1804; Neckera subgen. Pilotrichum (P. Beauv.) Chevall., Fl. Gén. Env. Paris 2: 71. 1827; Daltonia sect. Pilotrichum (P. Beauv.) Am., Mém. Soc. Linn. Paris 5: 295. 1826, nom. illeg.
Hookeria sect. Scleroneuron Hampe, Linnaea 20: 96, 97. 1847.
Callicosta Mull. Hal., Linnaea 21: 188. 1848.
Eupilotriclmm Müll. Hal., Bull. Herb. Boissier 65: 205. 1897, nom. inval.
Discussion. Pilotrichum is characterized by its ± frondose habit from a creeping stem, and its leaves with double, often crested costae and relatively short, usually prorulose cells. The setae are very short.
Pilotrichum was admirably monographed by Crosby (1969), and this treatment agrees completely with the species concepts presented there, and has relied on many of the measurements provided. However, my key is original, as Crosby’s is difficult to use because it relies on cellular features sometimes not present and ever-difficult phyllotaxy.
Although Crosby (1969) in his monograph thought Pilotrichum was near Helicoblepharum, I am unconvinced. Rather, I think the genus not too far removed from Callicostella. Admittedly, this placement seems dubious when one thinks of the strongly frondose, multi-pinnate species of Pilotrichum. However, I interpret these as the most advanced species of the genus, and not the ones to look at when speculating on phylogeny. Rather, I prefer to think of the lanceolate-leaved, pinnate but not erect. Lesser Antillean species of Pilotrichum as the least specialized. These taxa are also the ones without cristate costae. These taxa are not very different from some Callicostella species, and match in many “technical” features such as stem anatomy, axillary hairs, and calyptral morphology.
Pilotrichum has in more recent years mostly gone under the name Callicosta. The nomenclatural confusion is a result of difficulties in interpreting the Code. Crosby (1968), by assuming Article 63 was retroactive, argued that Pilotrichum R Beauv. is illegitimate because it contains the lectotype of Leptodon. Wilbur (1969) argued that Art. 63 was not retroactive and thus Pilotrichum did not need to be conserved. Subsequently, Crosby's conservation proposal was rejected (Florschütz, 1973). Finally, the 1988 Code specifically addressed the ambiguities in Art. 63, stating that a later lectotypification cannot make an earlier name illegitimate (Note 2). Thus, Pilotrichum P. Beauv. stands.