AbstractThe physiological response ofHylocomium splendens(Hedw) Br. Eur. to incubation in distilled water and solutions of phosphate, arsenate, selenite and selenate was investigated. Potassium efflux was found useful for comparing toxicity but was unreliable for short-term studies due to an initial rapid loss of potassium within ten minutes of immersion in any solution. This potassium loss could, under noninhibitory conditions, be regained by moss shoots. Selenite prevented re-accumulation of potassium lost in the first few minutes of incubation whilst arsenate promoted potassium loss throughout incubation. Photosynthetic oxygen evolution was progressively reduced under conditions of increasing arsenic uptake. Thus, photosynthetic rates were a more sensitive measure of anion toxicity than potassium efflux.Electron microscopic observations revealed that incubation in distilled water had little effect on immature leaves ofH. splendenswhilst arsenate incubation induced loss of cytoplasmic electron density and swelling of chloroplast lamellae in immature tissue. Mature leaves, incubated in both arsenate and distilled water, showed changes in celluar morphology compared to non incubated tissue. These changes were not reflected in potassium efflux data. The results indicate that shortterm submersion is disruptive to the fine structure of moss shoots. This is discussed in terms of the design and interpretation of experiments involving toxic ions.