The work of Collander (4, 5) illustrates that plant species and varieties are clearly distinct from one another in regard to the absorption of ions from nutrient solutions containing equal concentrations of several bases. The findings of Haas (14) and Smith et al. (26) indicate that the accumulation of ions in citrus is complicated by the fact that most commercially grown citrus trees are composed of two species or varieties of citrus: 1. a scion, or top, such as the Valencia orange, budded onto 2. a seedling rootstock, such as sour orange. These workers found highly significant differences in concentrations of nitrate, potassium, calcium, magnesium, manganese, copper, boron, zinc, and iron ions in the leaves of the scion that are attributable to a rootstock influence. Likewise, differences in concentrations of ions in two scion varieties of citrus growing on the same variety of rootstock were found attributable to scion influence (14). The work of Eaton (8, 9) has shown a selective absorption by plants of toxic amounts of such ions as chloride, sulphate, sodium, and boron introduced into soil with irrigation water. He found this selective absorption to vary for different species of plants. Haas (12) showed that Valencia orange trees on sweet orange rootstock, when growing in a nutrient solution to which large amounts of NaCl had been added, accumulated large quantities of chloride. Reed and Haas (20) also found that Valencia orange trees on sweet orange rootstock absorbed only small quantities of sodium when grown in nutrient solutions containing large amounts of NaCl. No results were reported, however, on the accumulation of these two ions in scions grown on other rootstocks. Both scion and rootstock influences on the accumulation of boron in citrus are shown by the work of Eaton and Blair (10). They worked with reciprocally grafted plants of Eureka lemon and Severinia buxifolia grown in controlled cultures receiving different amounts of boron. They found that the rootstock tended to control the quantity of boron accumulated in the scion. Other workers (13, 23, 26), who used other varieties of citrus as scion and rootstock, confirmed these results. A recent report (6) from this laboratory presented data on the injury to citrus trees of four rootstock-scion combinations subjected to varying amounts of added salt and boron, alone and combined, applied to a soil substrate. The test plants consisted of Valencia orange scions and Shary Red grapefruit scions on sour orange and Cleopatra mandarin rootstocks.