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BFNA Title:
Riellaceae |
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. |
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