Flat forms of Lemna gibba (fig. 1, a—c) occur rather commonly and differ in several characters from L. minor (fig. 1, d—e), e.g.: The largest diameter of the air cavities below the lower surface of the discs averages 0,19 mm in L. minor, 0,36 mm in flat growth forms of L. gibba and 0,47 mm in swollen growth forms of the latter. In L. gibba the walls of the cavities are thicker than in L. minor. L. minor is usually more convex above and the shape of the discs is more elongate than in L. gibba. L. minor is bright green, L. gibba more greyish green suffused with reddish purple. Pigmentation, if present, starts at the edges of the discs in L. minor and in the centre of the upperside in L. gibba. Discs of L. minor are often found in groups of 5 or more (up to 32), in flat L. gibba this forming of groups of young discs is less distinct, though not rare (up to 8). L. minor occurs in relatively poor waters, especially after pollution with substances of an organic nature (such as drain water and manure). This species appears to be faithful to the Lemno-Spirodeletum. It also occurs in the Ricciocarpo-Lemnetum. L. gibba prefers more eutrophic habitats, a relatively high chloride content and a fairly considerable specific conductivity. It thrives in fresh water especially after pollution with substances of an inorganic nature, such as industrial wastes and fertilizers. In habitats with a low chloride content this species may be associated with Wolffia arrhiza. The flat growth forms of L. gibba are either winterforms, young stages or forms of less favourable habitats, i.e. of relatively poor and of brackish waters.