Fissidens

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.

Distribution

Worldwide, mainly tropical areas.

Discussion

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 90°, 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).

Selected References

None.

Keys

1	Leaves ecostate or essentially so.	Fissidens hyalinus
1	Leaves costate	> 2
2	Leaves elimbate	Key A
2	Leaves limbate	> 3
3	Limbidium on all laminae	undefined
3	Limbidium confined to vaginant laminae	> 4
4	Laminal cells smooth, plane, sometimes bulging	undefined
4	Laminal cells mammillose or pluripapillose	Key D

Key A. Leaves costate, elimbate

1	Laminal cells mammillose or papillose	> 2
1	Laminal cells smooth, often bulging but not mammillose	> 7
2	Laminal cells mammillose or 1-papillose	> 3
2	Laminal cells pluripapillose (best seen on vaginant laminae)	> 5
3	Mammillae present, ± restricted to vaginant laminae.	Fissidens taxifolius
3	Mammillae or papillae on all laminae	> 4
4	Marginal cells of dorsal and ventral laminae not differing greatly in shape from inner cells; marginal cells of vaginant laminae coarsely and irregularly serrate.	Fissidens serratus
4	Marginal cells on dorsal and ventral laminae mostly oblate, differing in shape from inner cells; marginal cells of vaginant laminae regularly to irregularly crenulate.	Fissidens littlei
5	Papillae restricted to corners of vaginant laminal cells, inconspicuous (best seen in transverse section of leaf).	Fissidens bushii
5	Papillae over lumina of cells, not restricted to vaginant laminae, conspicuous	> 6
6	Leaf apex absent a clear, sharp cell.	Fissidens pallidinervis
6	Leaf apex ending in a clear, sharp cell.	Fissidens elegans
7	Laminal cells regularly pluristratose; plants usually coarse to the touch, usually found in rapidly running water.	Fissidens grandifrons
7	Laminal cells 1- or 2-stratose in patches, never regularly pluristratose; plants not coarse to the touch; found on various terrestrial substrates or aquatic	> 8
8	Leaves often 10 or more times as long as wide, fragile when dry; plants feathery in appearance, most often profusely branched; plants usually in quiet waters	> 9
8	Leaves usually less than 10 times as long as wide, not fragile when dry; plants not feathery in appearance, unbranched to moderately branched; plants on various terrestrial substrates	> 10
9	Costa ending 15-35 cells before leaf apex; sporophytes 1-5, axillary; capsules emergent; peristome teeth reduced, usually truncate.	Fissidens fontanus
9	Costa ending 5-15 cells before leaf apex; sporophytes 1-2, terminal; capsules exserted; peristome teeth complete, undivided or divided.	Fissidens hallianus
10	Distal part of costae obscured by overlying chlorophyllose cells.	Fissidens subbasilaris
10	Costae conspicuous throughout length, not covered by overlying chlorophyllose cells	> 11
11	Leaves in transverse section showing an oblongifolius-type of costa in proximal half of leaf	> 12
11	Leaves in transverse section showing either a taxifolius-, bryoides-type or reduced costa in proximal half of leaf	> 13
12	Plants usually robust; leaf apex rounded to broadly acute; minor laminae of many leaves rounded distally and free; medial marginal cells of vaginant laminae elongate, oriented obliquely; costa in distal part of leaf in transverse section showing a single row of enlarged cells.	Fissidens asplenioides
12	Plants small; leaf apex abruptly narrowed, rounded-obtuse; minor lamina ending near leaf margin; medial marginal cells of vaginant lamina not differing from inner cells; costa in distal part of leaf in transverse section showing 4 enlarged cells arranged in 2 rows.	Fissidens santa-clarensis
13	Plants 10-50 mm; costa taxifolius-type	> 14
13	Plants smaller, 0.6-4.5 mm.; costa bryoides-type or reduced	> 18
14	Laminal cells commonly 2-stratose in patches; leaf apex unevenly serrate.	Fissidens dubius
14	Laminal cells 1-stratose, rarely 2-stratose in patches; leaf apex regularly crenulate to serrulate to ± denticulate	> 15
15	Perichaetia and sporophytes terminal; rhizoids papillose.	Fissidens osmundioides
15	Perichaetia and sporophytes on short axillary branches, usually near distal end of stem; rhizoids smooth	> 16
16	Leaf margin entire but denticulate at leaf apex; leaf apex obtuse to broadly acute, usually bluntly mucronate.	Fissidens polypodioides
16	Leaf margin crenulate to regularly serrulate; leaf apex mostly acute	> 17
17	Plants to 85 mm; costa ending 2-3 cells before apex to percurrent; laminal cells 10-20 µm.	Fissidens adianthoides
17	Plants to 17 mm; costa ending several cells before leaf apex to excurrent, even in same plant; laminal cells 6-15 µm.	Fissidens aphelotaxifolius
18	Leaf apex rounded to obtuse.	Fissidens obtusifolius
18	Leaf apex acute, obtuse-apiculate or obtuse-mucronate	> 19
19	Juxtacostal cells in proximal parts of vaginant laminae enlarged, oblong, pellucid	Fissidens amoenus
19	Juxtacostal cells in proximal parts of vaginant laminae not greatly enlarged, oblong, nor pellucid	> 20
20	Vaginant laminae with 2-3 rows of irregularly elongate intralaminal cells; protonemata persistent.	Fissidens exilis
20	Vaginant laminae absent rows of irregularly elongate intralaminal cells; protonemata not persistent	> 21
21	Costa percurrent; minor lamina of all leaves narrowed distally, ending on costa.	Fissidens closteri
21	Costa ending before apex; minor lamina usually ending ± the distance between costa and margin or nearly so	> 22
22	Costa ending 3-7 cells before leaf apex; medial laminal cells 14-18 µm, laminal cells guttulate.	Fissidens pellucidus
22	Costa ending 6-15 cells before leaf apex; medial laminal cells 10-54 µm; laminal cells eguttulate.	Fissidens pauperculus

Key B. Leaves costate, limbate on all laminae, laminal cells smooth, plane to bulging

1	Stems dimorphic	> 2
1	Stems monomorphic	> 6
2	Limbidium confluent at leaf apex with costa, well developed on most leaves; limbidial cells 2- to 3-stratose; peristome typical bryoides-type.	Fissidens curvatus
2	Limbidium ending before leaf apex, best developed on perichaetial and subtending leaves, often confined to vaginant laminae; limbidial cells 1-stratose; peristome bryoides-type or anomalous	> 3
3	Peristome anomalous, teeth papillose distally, undivided, imperfectly divided or divided 4/5 their length, even in same capsule.	Fissidens scalaris
3	Peristome bryoides-type	> 4
4	Laminal cells small, 6-11 µm, bulging, obscure, ± arranged in discernable rows distally	Fissidens sublimbatus
4	Laminal cells larger, 6-18 µm, ± plane, distinct, not arranged in discernable rows distally	> 5
5	Dorsal lamina typically ending well above insertion; limbidium often confined to vaginant laminae of leaves other than perichaetial leaves; the Arctic.	Fissidens arcticus
5	Dorsal lamina typically ending at or very near insertion; limbidium variable, absent at times on some leaves; Arkansas, California.	Fissidens taylorii
6	Limbidial cells 2- to pluristratose; plants in running water	> 7
6	Limbidial cells 1-stratose, sometimes 2- to 3-stratose; plants not commonly found in running water	> 8
7	Limbidium confluent with costae in leaf apex; capsules exserted; eastern North America	Fissidens appalachensis
7	Limbidium ending a few cells before leaf apex; capsules emergent; Pacific Coast, Idaho	Fissidens ventricosus
8	Leaf apex obtuse to rounded, not apiculate.	Fissidens obtusifolius
8	Leaf apex neither rounded nor obtuse, if obtuse then apiculate, leaves mostly lanceolate to oblong	> 9
9	Laminal cells 6-18 µm, ± plane, distinct	> 10
9	Laminal cells small, 6-11 µm, bulging, ± obscure	> 11
10	All cells in vaginant laminae ± similar, neither greatly enlarged, nor ± oblong, nor pellucid	Fissidens bryoides
10	Groups of juxtacostal cells in proximal parts of vaginant laminae greatly enlarged, oblong, pellucid.	Fissidens zollingeri
11	Distal laminal cells not arranged in discernable rows; plants unbranched.	Fissidens minutulus
11	Distal laminal cells arranged in discernable rows; plants usually branched	> 12
12	Dorsal lamina ending near insertion to ± decurrent; limbidium on most leaves	Fissidens crispus
12	Dorsal lamina ending well above insertion; limbidium best developed on perichaetial and distal leaves, usually confined to vaginant laminae on infertile stems	Fissidens sublimbatus

Key C. Leaves costate, limbate; limbidium confined to vaginant laminae; laminal cells smooth, plane, sometimes bulging

1	Stems dimorphic	> 2
1	Stems monomorphic	> 4
2	Peristome anomalous, teeth papillose distally, smooth proximally, undivided, imperfectly divided, or divided 4/5 their length, even in same capsule.	Fissidens scalaris
2	Peristome bryoides-type	> 3
3	Laminal cells 6-18 µm, ± plane, distinct, not arranged in discernable rows distally	Fissidens arcticus
3	Laminal cells 6-11µm, bulging, ± obscure, ± arranged in discernable rows distally	Fissidens sublimbatus
4	Limbidium restricted to perichaetial leaves	> 5
4	Limbidium present on perichaetial and cauline leaves	> 6
5	Juxtacostal cells in proximal part of vaginant laminae not conspicuously enlarged nor pellucid	Fissidens bryoides
5	Juxtacostal cells in proximal part of vaginant laminae conspicuously enlarged, ± oblong, pellucid	Fissidens amoenus
6	Plants usually floating in quiet waters; leaves sometimes 10:1.	Fissidens hallianus
6	Plants usually not aquatic, but sometimes inunduated by fluctuating water levels; leaves less than 10:1	> 7
7	Proximal half of leaf in transverse section showing oblongifolius-type costa; minor lamina usually rounded and free distally in most leaves.	Fissidens asplenioides
7	Proximal half of leaf in transverse section showing bryoides-type costa; minor lamina acute, attached along entire distal end	> 8
8	Leaf apex neither rounded nor obtuse, if obtuse then apiculate; leaves mostly oblong to lanceolate.	Fissidens bryoides
8	Leaf apex obtuse to rounded; leaves ovate to oblong.	Fissidens obtusifolius

Key D. Leaves costate, limbate; limbidium confined to vaginant laminae; laminal cells mammillose, 1-papillose, or pluripapillose

1	Laminal cells pluripapillose	> 2
1	Laminal cells mammillose or 1-papillose	> 3
2	Leaf apex not ending in a clear sharp cell.	Fissidens pallidinervis
2	Leaf apex ending in a clear sharp cell.	Fissidens elegans
3	Limbidium confined to perichaetial leaves; leaf margin, other than limbate parts, serrate, often coarsely so on vaginant laminae.	Fissidens serratus
3	Limbidium on most leaves; leaf margin, other than limbate parts, crenulate-serrulate to serrulate	> 4
4	Limbidium along entire length of vaginant laminae, sometimes extending a short distance onto proximal part of adjoining ventral lamina.	Fissidens submarginatus
4	Limbidium confined to proximal 1/3-1/2 of the vaginant laminae.	Fissidens leptophyllus