1. Three crops of tomatoes were grown: one in sand culture on calcium concentrations of 0, 40, 80, and 160 p.p.m. and two-one sand, one water culture-on calcium concentrations of 0 and 160 p.p.m. for periodic harvesting. In the latter crops some plants were transferred after various intervals on 0 to 160 p.p.m. for observation of recovery effects. 2. Plants on 0 p.p.m. calcium showed typical deficiency symptoms: death of apical meristems and leaf margins, chlorosis of young leaves, and an enlargement and dark-green coloration of mature leaves. 3. Growth, as measured by height and leaf number, was limited in deficient plants but increased when such plants were transferred to nutrient containing calcium. 4. Calcium deficiency resulted in maturation and/or necrosis of apical meristems. Nuclei showed a reduction in size and an accumulation of large, probably chromatin, granules. In recovery plants the nucleus again increased in size, and the granules decreased in size and number. In root apices the changes in nuclear size were apparent even after 4 hours. The ratio of size of cell to that of nucleus was inversely related to calcium supply. 5. At 40 p.p.m. calcium the floral buds were more advanced, and there was a lower ratio of vegetative apices to floral apices than at 160 p.p.m. 6. Total cross-sectional area and percentages of this area contributed by the stele, vascular tissue, cortex, and pith were determined for internodes and hypotocyls, and total area and percentage stele for midribs and roots. Total cross-sectional area and percentage stele of internodes and hypocotyls were positively correlated with concentrations of calcium supplied. On plants grown on 40 p.p.m. calcium for 10 days before transferring to 0 p.p.m., these organs had the greatest cross-sectional area. This, plus reports in the literature, suggests that calcium concentrations below 40 p.p.m. may result in increased cell enlargement. 7. Calcium-deficient plants tended to have a higher percentage dry weight than did controls. 8. The critical level of calcium-that level below which cells become necrotic-in shoot apices was found to be 0.16-0.17% calcium on a dry-weight basis or 0.018-0.020% on a fresh-weight basis. The critical level of calcium in internodes appeared to be below that of shoot apices.