Plants: pale to dark green, sometimes becoming brownish to blackish, particularly in older parts, scattered to forming dense mats. Stems: monomorphic or dimorphic, erect, usually becoming decumbent, unbranched and branched; axillary hyaline nodules present or absent, when present composed of 1–3 enlarged, bulging, hyaline cells arranged linearly (larger, multicellular, protruding); epidermal and subepidermal cells small, incrassate, pigmented or not, or enlarged, thin-walled, hyaline; cortical cells larger, thin-walled, hyaline; central strand present or absent; rhizoids tan to reddish, basal and axillary, usually smooth, infrequently papillose; axillary hairs 1-seriate, filiform. Leaves: in few to numerous pairs, pinnately or palmately arranged, changing little to strongly crispate when dry, ovate to linear-lanceolate; vaginant laminae mostly acute, equal, ending on or near margin, or unequal, minor lamina ending between costa and margin, or, particularly in perichaetial leaves, rounded and free distally or narrowed and ending on or near costa; margin entire to serrate, marginal cells often differentiated into a limbidium; costa usually distinct, infrequently obscured, variable in length, absent or nearly so to short-excurrent, variable in structure (bryoides-type, oblongifolius-type, taxifolius-type); laminal cells usually eguttulate, rarely guttulate, 1-stratose, or 2-stratose in patches, rarely 3- or more stratose, smooth, plane, bulging, mammillose, 1-papillose, or pluripapillose, small, firm-walled, rounded to irregularly hexagonal, changing little when dry, to large, thin-walled, hexagonal to oblong cells, usually shrunken when dry [rarely prosenchymatous]. : Specialized asexual reproduction uncommon [by globose, multicellular, subterranean gemmae (tubers) or axillary, stalked, multicellular, clavate or filiform gemmae], rarely by chlorophyllose, branched filaments at bases of leaves. Sexual: condition dioicous, autoicous, or rarely synoicous. Perigonia: gemmiform, axillary, or at bases of stems, or sometimes scattered among persistent protonemata on substratum, or terminal on longer stems. Perichaetia: terminal on main stems and long branches, or terminal on short axillary branches. Seta: 1–2 [rarely more], smooth [papillose], straight or flexuous, yellow when young, darkening with age, or reddish. Capsule: usually exserted, theca erect or infrequently ± inclined, infrequently arcuate, radially or bilaterally symmetric, ovoid to cylindric, smooth, usually stomatose, rarely estomatose, stomata few, in proximal part of theca, phaneroporous; exothecial cells quadrate to oblong, longitudinal walls often thicker than cross walls, frequently collenchymatous; annulus none, abscission zone present; peristome variable; operculum conic, short- to long-rostrate. Calyptra: mostly cucullate, infrequently mitrate, smooth or prorate. Spores: smooth to finely papillose. Worldwide, mainly tropical areas.
Species ca. 450 (37 in the flora). Stems of a few species of Fissidens (F. arcticus, F. curvatus, F. scalaris, F. sublimbatus, F. taylorii) are here described as dimorphic: the fertile (perichaetial) stems are conspicuously shorter with fewer pairs of leaves than the infertile stems. Axillary hyaline nodules are arrested branch primordia, discussed in detail by Z. Iwatsuki and R. A. Pursell (1980). These structures are weakly developed in species found in the Western Hemisphere, but are well developed in some African and Asian species. Most species in the flora area have small, incrassate epidermal and subepidermal cells that are usually pigmented. One species, F. hyalinus, however, has large, thin-walled, hyaline epidermal and subepidermal cells that collapse when dry. Most species in the flora area have pinnately arranged leaves on elongate stems. Short-stemmed expressions of F. amoenus, F. closteri, F. exilis, F. hyalinus, F. serratus, and F. zollingeri have leaves palmately arranged. In the majority of species in the flora area the vaginant laminae where joined at their distal ends form an angle of less than 90Â°, a condition referred to as acute. In most species the vaginant laminae are unequal in size, the smaller of the two is the minor lamina, while the larger is the major lamina, which appears to be a continuation of the ventral lamina. Vaginant laminae are equal in size when their juncture ends on the leaf margin. The vaginant laminae can be fused along the entire length of their distal ends, or the distal end of the minor lamina can be rounded and free (as in Fissidens asplenioides) or narrowed toward the costa, ending on or near the costa (as in F. exilis). These last two conditions are particularly common in perichaetial leaves. Marginal leaf cells in some species in the flora area are differentiated into a limbidium, a band of 1-stratose to multistratose, hyaline to yellowish, prosenchymatous stereid cells. A stereid cell is elongate, typically has sharply pointed ends, and has a wall thicker than the diameter of the lumen. The limbidium is expressed on all laminae in Fissidens appalachensis, F. bryoides, F. crispus, F. curvatus, F. hyalinus, F. minutulus, F. ventricosus, and F. zollingeri. In F. arcticus, F. obtusifolius, F. scalaris, F. sublimbatus, and F. taylorii the limbidium is developed best on the vaginant laminae but can be found on the other laminae, particularly in the more robust leaves. In F. amoenus, F. elegans, F. hallianus, F. leptophyllus, F. pallidinervis, and F. submarginatus the limbidium is essentially restricted to the vaginant laminae, sometimes only of perichaetial leaves. Fissidens asplenioides is typically elimbate, but a short limbidium occurs on the vaginant laminae at times. Fissidens pellucidus and F. serratus, although generally elimbate, infrequently have a limbidium on the vaginant laminae of perichaetial leaves. Leaves of F. adianthoides and F. dubius generally show a marginal band of lighter cells that differ from other laminal cells only in depth and slightly thicker walls. This band of lighter cells should not be interpreted as a limbidium. Studies of transverse sections of costa (M. A. Bruggeman-Nannenga 1990) have resulted in the recognition of three basic costal types: bryoides-, oblongifolius-, and taxifolius-types. All are represented in the flora. The oblongifolius-type (see illustration of Fissidens asplenioides) is characterized by three stereid bands (two lateral and one adaxial) in the proximal part of leaf. There are as many as 16 peripheral guide cells and as many as five large central cells separating these stereid bands. In the bryoides- and taxifolius-types (see illustrations of F. pallidinervis and F. taxifolius, respectively) there are two stereid bands (both lateral) in the proximal part of leaf. However, in the taxifolius-type there are four or more peripheral guide cells and as many as five large central cells separating the stereid bands, while in the bryoides-type there are two peripheral guide cells and usually just a single large central cell. One species in the flora, F. exilis, has costa reduced in structure and another species, F. hyalinus, is essentially ecostate, the costa represented by a proximal vestige. Fissidens pellucidus is the only species in the flora area in which small, clear, dotlike structures called guttulae (E. S. Salmon 1899) are present in the laminal cells. The nature of guttulae is not known. These guttulae can easily be mistaken for papillae. The surface of laminal cells in Fissidens is variable: smooth and either plane or bulging, mammillose, 1-papillose, and pluripapillose. A smooth, plane surface is flat while a smooth, bulging surface protrudes slightly outward. A mammillose cell can be difficult to distinguish from a 1-papillose cell; both have a single hollow protuberance, but the former is more or less rounded while the latter is sharp-pointed. The presence of a few small, localized thickenings characterize a pluripapillose cell wall. The seminal study by B. H. Allen (1980), followed by the study by M. A. Bruggeman-Nannenga and W. Berendsen (1990), established that variations in peristome teeth are useful in determining natural relationships in Fissidens. Of the different peristome types recognized, the bryoides-, scariosus-, taxifolius- and similiretis-types are represented in the flora. In the bryoides- and taxifolius-types the trabeculae on the exterior surface of the undivided parts are higher and distinct from the lamellae. On the other hand, the trabeculae on the exterior surface of the undivided parts in the scariosus- and similiretis-types are indistinct from the equally high or higher lamellae. Variations also occur in the ornamentation in the area of bifurcation. In the taxifolius-type this ornamentation on the exterior surfaces of the undivided parts changes gradually, while in the other types there is a sudden change. The distal ends of the filaments in the bryoides- and scariosus-types are spirally thickened while in the similiretis-type they are squamose. The distal ends of the filaments in the taxifolius-type have protruding (nodose) trabeculae with spiral or vertical lamellar thickenings. In addition to these types there are anomalous peristomes that do not fit into any of the types described. Capsule in the majority of species in the flora are exserted on elongate setae. However, in F. fontanus, F. hallianus, and F. ventricosus the setae are short so that the capsules are emergent, not extending beyond the perichaetial leaves. Hedwig, obviously impressed with the similarity of the leafy stems of Fissidens with the fronds of ferns, coined several specific epithets reflecting this likeness. Thus, adianthoides, asplenioides, osmundioides, and polypodioides allude to the fern genera Adiantum, Asplenium, Osmunda, and Polypodium, respectively. The specific epithet taxifolius alludes to similarity with the leaves of Taxus (yew).