'the unique advantages of the Lemnaceae (duckweeds) as experimental organisms include the ease with which flowering can be affected by changes in the composition of the medium (4,5,(7)(8)(9)(10)(11)(12)(13). This characteristic becomes particularly important as these plants are increasingly employed not only in studies of photoperiodism, but also as assay systems for preparations having florigenic activity (e.g., 2).Posner (12,13) The results of two experiments comparing various concentrations of Hutner's and KNO3 media are summarized in Table I; they indicate clearly that the inhibition caused by progressively diluting Hutner's medium under these conditions cannot be observed in KNO3 medium. Vegetative growth in the two different media was indistinguishable in terms of the general appearance of the fronds, but frond number in the KNO medium cultures was generally somewhat lower than in the others.To determine whether the operative difference between the KNO3 and Hutner's media is indeed the presence of ammonium ion, experiments were conducted in which 0.1 X strength KNO3 medium was supplemented with various levels of NH4CI, (NH4)2SO4, NH4NO3 or KNO3. Two such experiments are shown in Table II, which makes it clear that ammonium ion from 1 X 10-' to 3 X 10-3 M is highly inhibitory under these circumstances.Kandeler's work on the effect of ammonium ion on L. gibba G3 (9) The sucrose-ammonium interaction on L. perpusilla is superficially reminiscent of the literature concerning carbohydrate-nitrogen interactions on flowering (1), but it differs in that here both substances are required for an inhibition to occur, rather than acting antagonistically. Kandeler (9-11) has already extensively reviewed the possible ways in which ammonium ion itself might act, e.g., by uncoupling phosphorylation or repressing the synthesis of nitrate reductase. However, no explanation of the relative specificity for flowering is forthcoming. An effect exclusively on photophosphorylation (9, 10) is ruled out by our frequent observation that L. perpusilla growing under nonphotosynthetic (5) conditions is still susceptible to the ammonium-sucrose-dilute medium inhibition of flowering. The possibility that this inhibition results from a level of amino acids inadequate for flowering has also been suggested (13), but again with no explanation for the specificity.The ammonium effect on L. perpusilla differs from that on L. gibba in its dependence on an external supply of utilizable carbohydrate and a low level of several major ions, notably calcium and phosphate (12). WVhether comparable interactions might be discovered in L. gibba is unknown, but clearly the situation in L. perpusilla is at present more susceptible to experimental analysis. This is particularly true since a number of common features indicate a close relationship between these phenomena and the observation (3) that incubation in distilled water daily for a few hours at a particular time of the inductive dark period specifically inhibits flowering in L. perpusilla. Such resul...