Lemna is a small, almost cosmopolitan and taxonomically complex genus of 13 species. Species can considerably vary within one season, the number of useful diacritic features to distinguish between species is limited and these characters are often poorly understood or not easily detectable. Moreover, in pressed specimens colouration disappears.
The number of veins per frond is a good character, especially to distinguish between Lemna minor and L. minuta, but these are quite often barely seen without magnification or preparation. Veins are usually best seen in decaying fronds or by holding the frond so that the light is transmitted to it. Finally, when boiled in ethanol or lactophenol fronds become colourless and translucent and veins become more apparent.
The presence or absence of anthocyanins (purple pigmentation) is a good feature as well and very typical of Lemna turionifera (much less so of the similar but native L. minor). The density of this pigmentation can vary from one frond to another. Pigmentation is frequently said to be reddish but, in fact (especially in the wild and without magnification), it rather appears to be brownish-purplish.
The production of turions (smaller, rootless fronds which are released from pouches at the margins of normal fronds) is usually regarded as an important feature (because unique of Lemna turionifera). As a matter of fact, turion-like fronds are exceptionally also produced in other species (even in native Lemna minor) and can be absent in L. turionifera (see Landolt 2000, Wolff & Bruinsma 2005).
In addition to the non-native species of Lemna treated below, some other taxa of Lemna and even of other closely related genera of the former Lemnaceae (especially Spirodela), have been reported as aliens in Europe (see for instance Ryman & Anderberg 1999 for a copiously illustrated overview). They are often very indistinct and could easily pass unrecorded. Lemna aequinoctialis Welwitsch could occur in Belgium as well – it is likely to be introduced through garden centers and nurseries. It is more or less intermediate between Lemna minor and L. minuta and is characterised by often elliptic fronds with 3 rather distinct veins. However, the most important distinguishing feature are the winged root sheaths and the sharp pointed root tips (always unwinged and rounded in Lemna minor) (see also Diekjobst 1984).
1. Fronds submersed (except at anthesis), more or less translucent, margins denticulate distally, base narrowed into a stalk-like portion. Fronds usually cohering in branched chains of numerous fronds (native) === Lemna trisulca
1. Fronds floating, opaque with entire margins. Fronds never with a distinct stalk, nor cohering in branched chains of numerous fronds === 2
2. Fronds strongly swollen (gibbous) on lowerside, up to 8 mm long. Veins usually (3-) 5(-7) (native) === L. gibba
2. Fronds plane or very slightly convex on lowerside, up to 5 mm long. Veins 1-3 (if more than 3, than outer ones branching from inner ones) === 3
3. Fronds elliptic (up to 2 times as long as wide), symmetric, usually less than 3 mm long, more or less pointed at apex, upper side with a sharp longitudinal ridge, vein 1 (often very indistinct). Purple pigmentation always absent === 1. Lemna minuta
3. Fronds ovate (slightly longer than wide), often very asymmetric, up to 5 mm long, broadly rounded at apex, upper side rarely with a longitudinal ridge, veins 3 (or apparently 5). Purple pigmentation often present === 4
4. Fronds almost always, at least in part, with purple pigmentation (usually more so on lower side), otherwise olive green. Turions frequently present, primarily in autumn === 2. Lemna turionifera
4. Fronds rarely with purple pigmentation (and then much less so on lower side), light green. Turions usually not produced (native) === L. minor
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