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By M. H. Grayum
English final draft: placed 1/May/2000
Landolt, E. 1986. Biosystematic investigations in the family of duckweeds (Lemnaceae) (vol. 2). The family of Lemnaceae---a monographic study, Vol 1. Veröff. Geobot. Inst. ETH Stiftung Rübel Zürich 71: 1-566.
5 gen. and ca. 36 spp.; cosmopolitan; 4 gen. and 6 spp. in CR.
Tiny to minute, annual, thalloid herbs, floating on or near the surface of standing or slowly moving fresh water, solitary or cohering in groups. Roots absent or 1-many, unbranched. Fronds simple, plane to subglobose, hyaline or membranous to subsucculent, linear to orbicular, 0.5-9.1 × 0.5-7.2 mm, with 0-16+ nerves, the margins entire or (rarely) dentate. Fls. usually one (rarely two) per frond, borne in lateral or dorsal pouches or cavities, naked or subtended by a scale-like, membranous leaflet (prophyll, or spathe). Stamens 1-2 (rarely 3), 2-or 4-locular. Pistil 1, compound; ovary superior, unilocular; ovules 1-2(5) per locule; placentation basal; style 1; stigma simple, funnel-shaped. Fr. a utricle with 1-4-seeded. Seeds obscurely reticulate (virtually smooth) or longitudinally ribbed. Reproduction mostly vegetative, via budding.
Easily distinguished from all other floating aquatics by their small size (< 1 cm). According to some interpretations, the Lemnaceae are monoecious, with the fls. of the present treatment corresponding to infls. each with a single, naked pistillate fl. and 1 or 2(-3) staminate fls. reduced to a single stamen each.
It is not uncommon to find two or more taxa of Lemnaceae growing intermixed, especially (in CR) in the Pacific lowlands. For example, Crow & Rivera 6015, from a lagoon near the town of Barranca, Prov. Puntarenas, includes Lemna aequinoctialis, Spirodela polyrhiza, and Wolffiella welwitschii.
The genus Wolffia has not yet been definitely recorded from Nic., CR, or Pan. It would key to Wolffiella below, but differs in having thickened, subglobose to cylindrical (as opposed to thin and planar) fronds.
1 Roots lacking; fronds without nerves, lacking raphides; infls. borne in cavities on upper surface of frond; stamen 1, 2-locular; seeds obscurely reticulate (virtually smooth)...Wolffiella
1' Roots present; fronds with at least one nerve, with raphides present; infls. borne in lateral pouches; stamens 2(3), 4-locular; seeds longitudinally ribbed.
2 Roots 1 per frond; fronds with 1-3(-5) nerves, lacking pigment cells; leaflet (prophyll) lacking at point of root attachment...Lemna
2' Roots (1)2 or (usually) more per frond; fronds with (3-)5+ nerves, possessing pigment cells; scale-like leaflet (prophyll) covering point of root attachment.
3 Fronds ca. 1.5-2× as long as wide, with 3-7 nerves; roots (1)2-7(-12), all perforating the prophyll; very rare, 1700-1800 m...Landoltia
3' Fronds 1-1.5× as long as wide, with 7-16+ nerves; roots 7-21, 1-5 (but not all) perforating the prophyll; 0-850 m...Spirodela
Les, D. H. & D. J. Crawford. 1999. Landoltia (Lemnaceae), a new genus of duckweeds. Novon 9: 530-533.
1 sp., South Amer., S Afr., SE Asia, E Asia, Austral. (introd. in North Amer., Eur., N Afr., N. Z.).
Landoltia punctata (G. Mey.) Les & D. J. Crawford, Novon 9: 532. 1999. Lemna punctata G. Mey., Prim. fl. esseq. 262. 1818; Spidrodela punctata (G. Mey.) C. H. Thomps.
Roots (1)2-7 per frond, to ca. 2 cm long, subtended by a scale-like leaf (prophyll). Fronds ca. 2.2-4.4 × 1.5-2.8 mm, elliptical to (usually) broadly or narrowly obovate, shiny above with medial row of small papules, often reddish below and on the margins, with 3--7 nerves; pigment cells present. Fls. borne in lateral pouches, subtended by a scale-like, membranous leaflet (spathe or bracteole); stamens 2, 4-locular, the outer locules extended above the inner; ovary with 1 or 2(--5) ovules. Seeds 0.8-1 × 0.5-0.6 mm, with 10-15 ribs. Turions not produced.
Presumably on ponds or in marshes, 1700-1800 m; vert. Pac. N Cord. de Talamanca (Candelaria region, Prov. Cartago). Fl. (?). (Gómez 19390; CR, INB, MO)
Landoltia punctata is morphologically intermediate between Lemna and Spirodela, and transitional to them evolutionarily. It is most likely to be confuse with Spirodela, but differs sharply from its congeners in its smaller, narrower fronds with fewer nerves and roots.
Landoltia punctata grows at the highest elevation of any Costa Rican Lemnaceae. The collection cited above (which bears no habitat data) is the only one known from the entire Mesoamerican region, between Texas and Colombia. Because this species is apparently native in S. Amer. and introduced in N. Amer., the status of the Costa Rican population is ambiguous. Like several other Lemnaceae (e.g., Lemna aequinoctialis), Spirodela punctata is a frequent commercial aquarium species, hence easily distributed.
13 spp.; cosmopolitan, New World distribution amphitrop.; 2 spp. in CR.
Roots 1 per frond, not subtended by a scale-like leaf (prophyll). Fronds lanceolate to ovate, membranous, with 1-3(-5) nerves; pigment cells absent; raphides evident. Fls. borne in lateral pouches, subtended by a scale-like, membranous leaflet (spathe or prophyll). Stamens 2 (very rarely 3), 4-locular, the outer locules elevated above the inner. Ovary generally 1 ovule. Seeds longitudinally ribbed. Turions not produced (CR spp.).
Lemna is most readily distinguished by the fact that each frond has a single root. It also typically has 1-3 nerves per frond, but these are less easily discerned.
1 Root sheath winged at base, the root-tips sharply pointed; fronds with three nerves; 0-850 m...L. aequinoctialis
1' Root sheath terete, the root-tips rounded to ± pointed; fronds with a single nerve; 600-1500 m...L. valdiviana
Lemna aequinoctialis Welw., Apont. 578. 1858 ('1859'). L. paucicostata Hegelm. ex A. Gray; L. perpusilla sensu Standley (1938), not Torr.; L. trinervis (Austin) Small.
Roots to ca. 3 cm long. Fronds narrowly to broadly obovate or elliptical, 2-6 × 1-3 mm, with three nerves. Anthers ca. 0.15 mm wide. Seeds oblong-obovoid, 0.4-0.5 × 0.3-0.4 mm, with 8-26 prominent ribs.
Shallow ponds, lagoons, canals, and swamps, in open or forested sites, 0-850 m. Throughout both slopes. Fl. Feb., May, Aug.-Oct. Worldwide in trop. and subtrop. regions, and widely introduced in temperate areas. (Grayum et al. 8704; INB, MO).
L. aequinoctialis is the most widespread and frequently collected species of Lemnaceae in CR. It was formerly included in L. perpusilla Torr., a very similar and closely related sp. now considered endemic to E North Amer. The veins can be difficult to see. Most CR collections of this sp. are fertile.
Lemna valdiviana Phil., Linnaea 33: 239. 1864. L. cyclostasa (Elliott) C. H. Thomps.; L. minima sensu Standley (1938), non Phil, ex Hegelm.
Roots to at least 1 cm long. Fronds narrowly oblong to obovate, ca. 2-4 × 0.7-2.0 mm, 1-nerved. Seeds 0.6-0.8 × 0.4-0.6 mm, with 15-29 ribs.
Shallow lakes and ponds, swamps and wet rocks, 600-1500 m; Atl. slope Cords. Central (Zarcero, Vara Blanca) and Talamanca (Lago Dabagri), Pac. slope Cord. Talamanca (Río Ceibo), S Fila Costeña (vic. of San Vito). Fl. ?. NE and W USA to Chile and Arg. (Gómez et al. 23200).
Lemna valdiviana occurs in wetter habitats and at higher elevations than L. aequinoctialis in CR. It is extremely similar to and often confused with the closely related L. minuta Kunth (including L. minima Phil. ex Hegelm. and L. minuscula Herter). The frond vein is very difficult to see. The CR material of L. valdiviana is sterile.
Ramírez-García, P. & A. Novelo R. 1989. Nota sobre Spirodela intermedia (Lemnaceae) en México y Costa Rica. Anales Inst. Biol. Univ. Nac. Autón. México, Bot. 59: 103-105.3 spp.
2 spp., cosmopolitan but centered in South Amer.; 2 spp. in CR.
Roots 7-21+ per frond, subtended by a scale-like leaf (prophyll). Fronds broadly obovate or subreniform to suborbicular, often ± thickened, with 7-16+ nerves; pigment cells present (best seen as brown dots in dead fronds); raphides present. Fls. borne in lateral pouches, subtended by a scale-like, membranous leaflet (spathe or prophyll). Stamens 2, 4-locular, the locules at ± the same level. Ovary with 1-2(-5) ovules. Seeds longitudinally ribbed. Turions produced or not.
Spirodela is easily recognized by its relatively large, many-rooted (usually 5+) fronds with 7+ nerves, and 7+ roots.. Spirodela apparently flowers less frequently than the other genera of Lemnaceae; all CR material appears sterile.
1 Prophyll (scale-like leaf covering point of attachment of roots) perforated by 2-5 roots; turions never present; upper surface of frond lacking red spot...S. intermedia
1' Prophyll perforated by 1(-2) roots; turions sometimes present; upper surface of frond often with a red spot above node...S. polyrhiza
Spirodela intermedia W. Koch, Ber. Schweiz. Bot. Ges. 41: 113. 1932. S. biperforata W. Koch.
Roots to at least 1.5 cm long. Fronds broadly obovate to subreniform, ca. 4-7 × 2-6 mm, uniformly green above, purplish below and on the margins. Seeds 1-2 × 0.8-1.2 mm, with 15-22 ribs. Turions not produced.
Coastal lagoon, rice-fields, undertow, 0-20 m; S Atl. slope (Laguna Río La Estrella, Prov. Limón), Pac. slope, from Río Grande de Tárcoles toward south. Fl. (?). Chiapas, El Salv., CR, Pan. to Arg. and Chile. (Lot et al. 1251, MO)
The geographic ranges of the largely South Amer. S. intermedia and the more northern S. polyrhiza overlap in Central Amer. The two spp. are only weakly distinguished. The specimen cited above agrees well with South Amer. material identified as S. intermedia, which differs in general aspect from S. polyrhiza in having larger and thicker (on the average) fronds, uniformly green on the upper surface and with more obscure venation. Most of the distinctions proposed by Ramírez-García & Novelo R. (1989) do not hold up, at least in the herbarium. The differences given in the above key are inconsistent, and the prophyll character is difficult to evaluate on dried material.
Spirodela polyrhiza (L.) Schleid., Linnaea 13: 392. 1839. Lemna polyrhiza L., Sp. pl. 970. 1753.
Roots to ca. 2 cm long. Fronds broadly obovate or subreniform to suborbicular, 3-6 × 2-5 mm, green above often with a purplish spot over the node, purplish below and on the margins. Seeds 0.7-1.0 × ca. 0.7 mm, with 12-20 ribs. Turions ± orbicular, ca. 1-1.3 mm diam., rootless.
Shallow lagoons, creeks, and rice-fields, 0-850+ m; S Atl. slope (Puerto Viejo de Talamanca), Pac. slope from Río Barranca to Río Térraba and Osa Pen. Cosmopolitan, but absent from the Atlantic drainage of South Amer. (Crow & Rivera 6013, MO).
Only subtly distinguished from S. intermedia, as discussed thereunder. According to Landolt (1986: 467), "the best characteristic for determining S. polyrhiza is its ability to form turions" (morphologically differentiated resting fronds). However, turions are produced only "under unfavourable conditions", and are evident in a few CR collections (i.e., Maas 7930).
9 spp. of subtrop. and trop. Amer. and Africa Pseudowolffia, Wolffiopsis; 1 sp. in CR.
Wolffiella welwitschii (Hegelm.) Monod, Mém. Soc. Hist. Nat. Afrique N., Hors Sér. 2: 229. 1949. Wolffia welwitschii Hegelm., J. Bot. 3: 114. 1865; Wolffiopsis welwitschii (Hegelm.) Hartog & Plas.
Fronds rootless, ca. 5-6 × 4-5 mm, suborbicular or rounded-quadrangular, hyaline, largely submerged; pigment cells present; raphides absent; daughter fronds emerging from a basal, triangular pouch with a medial, ventral row of elongated cells. Fls. 2 per frond, borne in paired dorsal cavities, not subtended by a scale-like leaflet. Stamen 1, 2-locular; anther ca. 0.1 mm wide. Ovary with 1 ovule. Seeds ± ellipsoidal, obscurely reticulate (virtually smooth), 0.4-0.5 × 0.3-0.4 mm.
Shallow lagoons, 0-20 m; N Pac. slope Prov. Guanacaste lowlands (N of Santa Cruz; Palo Verde) S to Río Barranca, Prov. Puntarenas. Fl. May, Sep.-Oct. Chiapas-Ecua., Ven. and Braz., WI, trop. Africa. (Crow & Rivera 7415).
The very thin fronds, lacking roots and nerves, readily distinguish Wolffiella from all other Lemnaceae. Two other spp. have been collected in S Central Amer.: W. lingulata (Hegelm.) Hegelm. (Canal Area of Pan.) and W. oblonga (Phil.) Hegelm. (Nic. and Pan.). Both differ from W. welwitschii in having a single fl. per frond and an oblique (inframedial) row of elongated cells on the ventral wall of the triangular pouch. Most CR material of W. welwitschii is fertile, hence most easily identifiable by the paired fls. and frs.; the medial row of elongated pouch cells is less conspicuous.