Since this paper was submitted to the editor, the paper by MoHargue and Calfee (Plant Physiol. 7: 697-703, 1932) has appeared. As their conclusions are in substantial agreement with the findings reported in this paper in regard to the essential nature of manganese for the growth of Lemna, the original manuscript was returned to me and shortened. It should be noted that in these experiments the Lemna were grown free from microorganisms, and that therefore the amount of manganese required for maintenance would be expected to be much lower than with the technique of McHargue and Calfee, as was the case. It has been previously reported (Clark and Fly 4) that it was not necessary to add manganese to solutions of purified salts in order to secure good reproduction of Lemna major, and that the addition of manganese to an inorganic nutrient medium in which Lemna grew well produced some effect upon the vegetation but little upon the rate of reproduction until toxic concentrations were reached. The possibility that extremely small amounts of manganese might be essential to the plants and be present in sufficient quantity even in salts recrystallized iseveral times, together with the failure of many workers to obtain growth in green plants without manganese, particularly of Hopkins (6) with Lemna minor, caused the problem to be investigated further. Lemna major (Spirodela polyrhiza) has been grown in these laboratories with inorganic salt solutions for several years. The nutrient solution used as standard consists of magnesium sulphate (24 mg. Mg per liter), potassium nitrate (312.8 mg. K), primary calcium phosphate (16.1 mg. Ca), and ferric chloride (0.6 mg. Fe), adjusted to pH 4.7-4.8 with KOH (3). The salts are recrystallized, usually three times. The water is three times distilled, the final distillation being in pyrex glass, and the 'solution in preparation and use is handled altogether in this glassware. The substitution of quartz Erlenmeyer flasks for pyrex made no change in the rate of growth of the plants. This medium has been used for the growth of Lemna minor by Ashby (1) at the University of London. Lemna has been found valuable for physiological experiments by Bottomley and Mockeridge in England, Olsen in Denmark, and by Saeger, Deuber, Wolfe, and others in this country.