Pea roots have been cultured in sterile media in the presence and in the absence of added iron, and the effects of deficiency have been studied in three series of experiments. In the first, roots grown in deficient and full nutrient media were taken at intervals and on each sample growth and metabolic measurements were made. In the second, roots in which the earliest growth effects of deficiency had been observed were dissected into successive centimetre segments and metabolic and growth measurements were made on the separate fragments. In the third series of experiments the effects have been analyzed of transferring a root from a deficient medium in which growth had ceased to a full medium. It has been shown that after culture for 7 days in the deficient medium, increases in length and number of cells virtually cease; after this stage also the increase in respiration is relatively small. On the other hand protein continues to increase throughout the whole cultured period of 11 days. Oxygen absorption has been analyzed into a cyanide insensitive and a cyanide sensitive fraction, and it has been found that in the absence of iron, whereas the cyanide insensitive fraction increases continuously from the third to the eleventh day, the sensitive fraction ceases to increase after the seventh day. When roots in which growth has ceased were transferred to a full medium growth was resumed. The deficient roots were therefore not moribund. The arrest in growth is due to an abrupt cessation in division that occurs at about 7 days, and all the evidence indicates that this is the result of a disturbance confined to the meristem. The arrest in division cannot be attributed to an inhibition in the synthesis of protein. Evidence is presented which shows that in normal circumstances amino acids are probably synthesized in expanding or mature zones of the root, that they are carried forward in a polar translocation stream, and that they condense with the formation of proteins in cells that are being formed by meristematic activity. In the absence of iron division ceases, but the synthesis of amino acids continues. The sumps into which these acids are normally discharged are no longer provided, and they therefore tend to condense to proteins in the more mature cells adjacent to the apex. The cyanide-sensitive fraction of respiration is attributed to the activity of a cytochrome oxidase system. There is little or no further increase in this system after the time at which division ceases, and the coincidence may indicate a causal relation between the two events. But at the time when division ceases the cytochrome system is apparently normal in the apex and adjacent regions of the root. Therefore, if the cessation of division is due to arrest in the synthesis of cytochrome the position would be that a minimal quantity of cytochrome is required in the formation of a cell, and that when the supply of labile iron has been reduced to the level at which this minimal quantity cannot be provided then division ceases. This interpretation is consistent with the observation that the cessation of division is abrupt. The evidence, however, is not sufficiently extensive to warrant this conclusion, and the data do not exclude the alternative hypothesis that the depression in cytochrome synthesis is a consequence of the arrest in division.