RIELLACEAE Engler, Syll. Pflazenfam. Grosse Ausgabe: 45. 1892
Sharon E. Bartholomew-Began
Plants submerged aquatics, erect or ascending, with the thallus axis (= stem) postically coiled, bearing a prominent dorsal lamina (= wing) on one side and lateral and ventral leaf-like scales on both sides, with the rhizoids clustered basally. Dorsal thallus wings undulate to ruffled (coiled), tapering toward axis base, 1-stratose, with the cells dimorphic; oil cells scattered. Rhizoids smooth-walled. Sexual condition dioicous or less commonly monoicous. Gametangia developed acropetally. Sporophytes reduced, with the capsule cleistocarpous.
Genus 1 (1 in the flora): nearly worldwide, submerged, aquatic.
SELECTED REFERENCE Schuster, R. M. 1992. The Hepaticae and Anthocerotae of North America, East of the Hundredth Meridian. Vol. V. Field Museum, Chicago.
1. RIELLA Montagne, Ann. Sci. Nat. III. 18: 11. 1852 • for DuRieu de Maisonneuve, suggesting a river or small brook
Duriaea Bory & Montagne, Compt. Rend. Hebd. Séances Acad. Sci. 16: 1115. 1843 (not Mérat); Maisonneuvea Trevisan
Plants 0.6--10 cm, branched
sparingly, bright green. Shoot apex falciform to
circinate. Oil cells reduced, with 1 oil body per cell. Leaf
scales dimorphic, in 2 lateral and 2 ventral rows along the axis. Specialized
asexual reproduction frequent, gemmae multicellular, ventral on the
axis. Antheridia marginal along the wing, sunken, single in
involucres. Archegonia alternating left and right along the wing axis,
single, in subglobose, pyriform to flask-like involucres. Sporophyte
seta abbreviated, not elongating; capsule wall 1-stratose; elaters absent but
with nurse cells interspersed with immature tetrads. Spores large, dissociated at maturity.
Species 17 (2 in the flora): submerged aquatic in semiarid and arid regions, nearly worldwide, but disjunct, sporadic and local throughout its range; w and sw North America, Mexico, South America, Eurasia, Africa, Australia.
Riella is unique among hepatics in being a submerged aquatic in fresh or brackish water of temporary pools or streams in arid and semiarid regions or rarely in permanent bodies of water. The gametophyte is intolerant of desiccation and capable of surviving for only a short time after water level subsidence. The viability of the genus is insured by spores, which are usually produced a few weeks before the water level subsides. The spores are adapted to survive many years of desiccation and still remain viable, e.g., up to 3 years in R. americana and 13 years in R. capensis (R. A. Studhalter 1932). Riella, then, is an annual hydrophyte that is capable of completing its entire life cycle in less than 3 months. Locating Riella populations is often difficult, as gametophytes are ephemeral and submerged. In fact, many populations of Riella have been discovered accidentally when Riella spores germinated from mud samples that had been collected for scientific purposes unrelated to bryology.
Very few species of Riella exhibit distinguishing vegetative characters. Therefore, taxonomic differentiation within the genus is primarily based on spore and involucre morphologies. Two subgenera have been defined based on involucre characters; Riella subg. Trabutiella Porsild. which has female involucres that bear longitudinally oriented wings (Porsild 1902) and Riella subg. Riella which has smooth, wingless female involucres. The North American species R. americana and R. affinis belong to the subgenera Riella and Trabutiella, respectively.
SELECTED REFERENCES Perold, S. M. 2000. Studies in the Sphaerocarpales (Hepaticae) from southern Africa. 3. The genus Riella and its local species. Bothalia 30: 125--142. Porsild, M. P. 1902. Sur une nouvelle espèce de Riella (subgen. nov.: Trabutiella) de l’Asie centrale. Bot. Tidsskr. 24: 323--327. Proctor, V. W. 1972. The genus Riella in North and South America: distribution, culture and reproductive isolation. Bryologist 75: 281--289. Thompson, R.H. 1940. A second species of Riella in North America. Bryologist 43: 110--111. Thompson, R.H. 1941. Morphology of Riella affinis. I. Germination of the spore and development of the thallus. Am. J. Bot. 28: 845--855. Studhalter, R. and M.E. Cox. 1941. The lateral leaf scale of Riella americana. Bryologist 44: 19--27. Studhalter, R. and M.E. Cox. 1941. The ventral scale of Riella americana. Bryologist 44: 29--40.
1. Female involucral flasks lacking lamellae (= ribs), plants dioicous …….1. Riella americana
1. Female involucral flasks with at least 8 longitudinal lamellae, plants monoicous ….. 2. Riella affinis
1. Riella americana M. Howe & Underwood, Bull. Torrey Bot. Club 30: 218. 1903
Plants 1.0--3 cm, caespitose, unbranched or dichotomously branched 1 to 4 times; shoot apex circinate. Axis elliptic, 0.2--0.8 mm diam. Dorsal wing 1.5--5 mm wide, margin entire, lacerate or erose with age, with wing cells near the axis 70--100 ´ 30--50 µm, with the marginal cells 30--50 ´ 22--40 µm. Oil cells 21--23 ´ 17--25 µm; oil bodies 15--17 µm diam. Lateral leaf scales remote, oriented obliquely, 1-stratose with a multistratose base, ovate to oblong (obovate to lingulate), 355--800 ´ 125--400 µm. Ventral leaf scales 1-stratose, irregular, often panduriform with a broad isthmus and equal or unequal lobes or oblong (obovate-ovate), 165--540 ´ 100--350 µm. Specialized asexual reproduction by gemmae, ventral from the youngest axis portions, intermixed apically with the scales, panduriform with two rounded lobes, 730--1050 ´ 450--775 µm, multicellular. Sexual condition dioicous. Antheridia up to 75 in a single series; antheridial body ca. 210--235 ´ 160--185 µm. Archegonial involucre subglobose to ovoid, 1.4--1.8 ´ 0.8--1.2 mm, with the mouth truncate or slightly pointed, subpapillose; lamellae absent. Sporophyte foot broadly ovate, ca. 130-- 250 ´150--200 µm; seta 100--200 µm; capsule globose, 0.5--1.3 mm. Spores spherical, 100--130 µm, with the distal face spinose, the spines 10--24 µm, occasionally curved, the apices somewhat truncate, dilated, acute, the basal membranes irregularly radiating, with the proximal face spinose, the spines conical, 3--6 µm, basal membranes rare or absent.
Submerged aquatic in temporary pools and streams in arid and semiarid areas; moderate elevations; Calif., N.Mex., S.Dak., Tex.; Mexico; South America.
Riella americana is known from Texas, New Mexico (San Miguel County and Luna County), California (Modoc County), and one location in South Dakota (near Brookings).
2. Riella affinis M. Howe & Underwood, Bull. Torrey Bot. Club 30: 221. 1903
Plants 0.6 -- 2.4 cm, caespitose, unbranched or branched sparingly; shoot apex falciform. Axis slightly flattened on dorsal side, 0.1--0.3 mm wide, mostly thin and flaccid. Dorsal wing 2.0--3.0 mm wide, margin entire, lacerate or erose with age, with the wing cells near axis 86--113 ´ 22--50 µm, with the marginal cells 25--48 ´ 25--35 µm. Oil cells 20--24 ´ 20--25 µm; oil bodies 15.0--17.5 µm. Lateral leaf scales remote, oriented obliquely, 1-stratose, linear-lanceolate, 500--730 ´100--160 µm. Ventral leaf scales 1-stratose, linguiform, lanceolate, or linear, often with median constriction, 250--400 ´ 160--180 µm. Specialized asexual reproduction unknown. Sexual condition monoicous, protandrous. Antheridia 1--7(13) in a single series; antheridial body ca. 120--160 ´ 90--128 µm. Archegonial involucre ovoid, 1.4--2.0 ´ 1.0--1.5 mm, with the mouth contracted and often subacute, lamellae 8, longitudinal, 0.1--0.2 mm broad, with the margin undulate-sinuate or subentire. Sporophyte foot nearly spherical, ca. 160--180 µm; seta 100--115 µm; capsule ± globose to ovoid, 750--800 µm. Spores ovoid, 80--120 µm, with the distal face spinose, the spines 6--13 µm, the apices truncate, slightly dilated, occasionally emarginate-2-fid, rarely acute, the basal membranes forming a few imperfect areolae, with the proximal face spinose, the spines truncate or obtuse, or with tubercules, ca. 2--5 µm, basal membranes absent.
Disjunct and sporadic in arid and semiarid regions; low elevations; Calif.; Eurasia; Africa.
Riella affinis is known only from a single location in North America; Lake Lagunita, on the campus of Stanford University, Palo Alto, California. The unusual distribution of R. affinis suggests that it may not be native to North America, but instead introduced either inadvertently or with purpose to the Stanford University campus (perhaps by D. H. Campbell). In addition to differences in involucre morphologies, the absence of gemmae in R. affinis is often cited as a useful character to differentiate between R. affinis and R. americana.