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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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