keys to the vascular plants of east texas - Botanical Research ...

722 LEMNACEAE/LEMNA
5. Fronds slightly to strongly convex or gibbous beneath; upper frond surface with papilla
at apex bigger than others; not forming turions ___________________________________ L. obscura
2. Fronds with 1 vein (use backlighting), without red or purple pigmentation; upper surface of
fronds flat, smooth or with obscure median papillae, without prominent papillae; roots 0.5–
1.5 cm long; species rare in East TX.
6. Fronds very asymmetrical at the base (= end of frond closest to attachment to other fronds
and to where root arises), 1.3–3 times longer than wide, mostly (1–)2.5–5 mm long; vein
reaching at least 3/4 of the distance from the node (= point where root attaches on lower
frond surface) to the apex, usually distinct; plants sometimes submersed _______________ L. valdiviana
6. Fronds nearly symmetrical at the base, 1–2 times longer than wide, mostly 1–3(–4) mm
long; vein reaching at most 2/3 of the distance from the node to the apex, sometimes
indistinct; plants always floating on the surface ______________________________________ L. minuta
Lemna aequinoctialis Welw., (of equinoctial zone, from equatorial regions), DUCKWEED, LESSER
DUCKWEED. Fronds floating, ovate to lanceolate, 1–6.5 mm long, 0.8–4.5 mm wide, 1–3 times longer
than wide, never pointed, nearly symmetrical, light to medium green, with 3 veins; lower surface
flat to slightly convex; distinct turions absent; roots 1–3 cm long, the wing of root sheath 1–2.5
times longer than wide. Widespread in e 1/2 of TX, also Trans-Pecos; throughout much of the s 2/
3 of the U.S. [L. trinervis (Austin) Small] This is the common DUCKWEED in much of East TX.
Lemna minuta Kunth, (minute, very small), LEAST DUCKWEED. Fronds floating, ovate to lanceolate,
0.8–4 mm long, 0.5–2.5 mm wide, 1–2 times longer than wide, light green, with 1 vein;
lower surface flat to slightly convex; turions absent; roots 0.5–1.5 cm long, the root sheath not
winged. Bowie, Colorado, Dallas, Hays, Marion, Smith (Landolt 1986), Polk, and Travis (Turner et
al. 2003) cos.; widely scattered in TX; widespread in the U.S., particularly in the sc and sw portions.
[L. minima Phil. ex Hegelm., L. minuscula Herter, L. valdiviana Phil. var. minima Hegelm.]
According to Landolt (1986), this species is sometimes difficult to distinguish from L.
valdiviana; allozyme studies indicate that the two are “closely related and recently diverged sister
species” (Crawford et al. 1996). These two species are considered the most morphologically
reduced in the genus Lemna (Landolt 1986).
Lemna obscura (Austin) Daubs, (hidden), DUCKWEED, LITTLE DUCKWEED. Fronds floating,
broadly ovate to suborbicular, 1–3.5 mm long, 0.8–3 mm wide, 1–1.5 times longer than wide;
green to yellowish green, typically with reddish purple pigmentation, especially on lower surface;
upper frond surface with a distinct papilla near apex and several smaller indistinct papillae
along midline; lower frond surface slightly to strongly convex or gibbous; roots 1–4(–15) cm
long, the root sheath not winged. Bowie, Fayette, Gonzales, Harris, Lamar, Titus, and Washington
(BRIT—identified by Landolt), Bastrop (Johnston 1972), Angelina, Austin, Dallas, Freestone,
and Orange (Turner et al. 2003) cos.; widespread in e 1/2 of TX; nearly throughout the e U.S. w
to SD and TX. [L. minor L. var. obscura Austin, L. minor of TX authors, not L.] The Lemma gibba–
minor–obscura–turionifera complex is quite confusing. However, according to Landolt (2000),
L. minor is not known from TX and L. gibba occurs only to the w of East TX. The name L. minor has
been misapplied to plants of L. obscura in TX in the past (e.g., Jones et al. 1997; Turner et al. 2003).
Lemna perpusilla Torr., (very weak and slender), DUCKWEED, MINUTE DUCKWEED. Fronds floating,
ovate to lanceolate, 1–4 mm long, 0.8–3 mm wide, 1–1.7 times longer than wide, light green,
with 3 veins; apical and central papillae prominent; distinct turions absent; roots 1–3.5 cm long,
the wing of root sheath 2–3 times as long as wide. Dallas Co. (Landolt 1986); apparently rare in
TX; Landolt (1986) gave only the Dallas locality, and the map in Landolt (2000) indicated that
the species occurs only in the extreme ne part of East TX; ne U.S. s to NC and w to KS and TX.
While not officially designated as such (e.g., TOES 1993; Carr 2002d; Poole et al. 2002), given its
limited distribution in the state, we consider this species to be of conservation concern in TX. �