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45Ca, applied to roots of apple seedlings, moved readily to the developing leaves. Kinetin, benzyladenine, and B sprays increased movement. NO,-as the source of N increased movement and accumulation of Ca into mature leaves; NH4+ increased movement into new leaves. Translocation in the stem is effected by a nonspecific ion exchange. Any divalent cation can free Ca for ascent. The exchange may be a property of lignin. Genetic differences in the uptake and translocation of Ca exist among apple seedlings. Those which show juvenile leaf characteristics translocate Ca into mature leaves more readily. Translocation of Ca in woody species appears to be similar to that reported for herbaceous plants.Sufficient Ca in the fruit may prevent the development of several physiological disorders of the apple (10, 29). Ca may be translocated into the fruit for a short time only at the beginning of the growth of the fruit, or throughout the entire growing season (39). Occasionally, Ca may even decrease in the fruit, perhaps being transported back into the tree (39).Ca movement in the tree must be understood before Ca translocation into the fruit can be influenced. We grew the seedlings in solution culture with vigorous aeration in 1-liter brown plastic bottles. The composition of the nutrient solutions are given in Table I. Prior to this time, the seedlings had been grown for 6 to 8 weeks in flats of quartz sand or in aerated solution culture receiving the intermediate level of Ca, and N as } NH4+ and T NO3-. The solutions were changed weekly until the plants reached the height of 25 to 30 cm. Then 12 ,uc of 45Ca were added to the solution. Seedlings were harvested 1 week after the addition of 45Ca. Appropriate tissue samples were prepared for counting, and autoradiographs were made of the whole plants.In the soil experiment we grew the seedlings for 2 years in 3-gallon crocks of a soil-sand-peat mixture with which the following quantities of salts had been thoroughly mixed at planting time: KH2PO4, 10.5 g; Mg(OH)2, 4.5 g; Ca(OH)2, 4.5 g; and either NaNO3, 18 g; NH4N03, 9 g; or (NH4)2S04, 15 g. To inhibit the conversion of NH4+ to NO3-, 0.06 g of 2-chloro-6-(trichloromethyl) pyridine was mixed with the substrate. During the second growing season only the N sources in solution were added to the crocks at the following weekly rates: NaNO3, 15 g; NH4NO3, 7.1 g; or (NH4)2SO4, 6 g plus NH40H(28%), 10 ml.Autoradiographs were prepared by using 8 x 10-inch Kodak no-screen ready-pack X-ray film. Envelopes of film were opened; plants previously mounted on cardboard and dried were placed in the envelope; the envelope was sealed with black plastic tape; and the film was exposed for 1 week.Samples for radioactivity were counted in a planchet-counting system equipped with a gas flow detector. Ca in the leaves was analyzed with an "emission" spectrometer. Details of this analysis have been described previously (11).We measured Ca exchange from the sites by the use of 5-cm stem pieces from plants grown in 45Ca. Stems were inverted and t...