Perhaps the earliest suggestion that soluble materials may make a circuit within a plant was made by Hartig (16). He concluded that materials assimilated in the leaves moved downward in the bark and were stored in the parenchyma and rays. In the spring these materials were brought into solution, passed into the trachea and ascended with the moving current of water. Atkins (1) expressed essentially the same views, and later the data of 'Mason and Maskell (20) indicated that such a circuit might be made without an intervening storage period. The latter investigators suggested that nitrogen, phosphorus, potassium, and other minerals ascended the stem primarily in the xylem, and any excess not currently used in the leaves was re-exported downward in the phloem. The ratios of N, P, and K to the carbohydrate moving downward in the phloem appeared to be in excess of that required for growth of the roots, so it was suggested that the excess was liberated into the xylem sap to re-ascend the stem. Biddulph (2) showed the rapidity of movement of phosphorus in the phloem of the bean plant and the possibility of a more or less continual circulation of this element throughout the plant. Helder (17) has confirmed the circulation of phosphorus in barley and bean plants. A free circulation of phosphorus is to be expected on the basis of its mobility in the phloem and its rapid metabolic turnover in many cellular reactions which serve to maintain a supply of inorganic phosphate in the cellular fluids.Prior to the work of Thomas et al (28) sulfur was considered to be relatively immobile in plants (Wood (29)), but the demonstration by Thomas et al of movement from the leaves on one stem of a multistemmed alfalfa plant downward to the crown and then upward in another stem, left little doubt of its mobility. Biddulph et al (5) showed sulfur to be freely mobile in the phloem of the bean plant, moving at rates comparable to other mobile substances. It is suspected that the earlier views of immobility may stem from the rapidity with which sulfur may be metabolically incorporated into newly formed protein, leaving little free to re-circulate. The present investigation is intended to test this concept.Calcium has been considered immobile in the phloem since the early investigations of the withdrawal of minerals from leaves prior to their abscission in the autumn (22). Bledsoe et al (9) recently provided a very direct demonstration of the immobility of calcium in the phloem of the peanut plant, and subse-

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