The effect of manganese on carbon assimilation, respiration, and translocation has been studied using isolated rooted potato leaves and small potato plants. Methods are described for the rooting and culture of the leaves and plants. It was found that normal potato leaves rooted readily when treated with α-naphtha-leneacetic acid (2 p.p.m.), but that very few of the manganese-deficient leaves produced roots, the critical level being about 15 p.p.m. manganese on a dry weight basis. The growth of isolated deficient leaves was also much less than that of control leaves, but in no case did characteristic manganese-deficiency symptoms develop, although the manganese level had fallen below that of leaves which showed symptoms when attached to the plant. A marked difference in net assimilation rate was found between leaves which had a high or low manganese content at the time of rooting. The addition of manganese after rooting to low manganese leaves did not, however, cause an increase in assimilation rate, although the manganese content of the leaves had been raised to that of the control leaves. Manganese was shown to have only a small effect on respiration, higher respiration rate being consistently associated with a higher manganese content; the addition of manganese to ‘deficient’ leaves did not cause any increase in respiration. No effect of manganese on translocation was detected.