IntroductionBoron is now included, by most plant physiologists, among the elements essential for the normal growth of higher plants.Until recently, most of the work has been done on the amount and habit of growth produced in many of our common plants, either by too great a quantity of boron or by a lack of it. BRENCHLEY (1) and COLLINGS (5) found boron to be toxic for both growth and germination with the concentrations which they used. Since then, by using very small amounts, BRENCH-LEY and WARINGTON (3), and others (9,10,12, 14,15, 16,18, 20,21,23, 24,26,27), have found boron to be indispensable for the normal growth of the plants used.Much less investigation has been done on the effect of boron on the internal structure of the plant. WEBBER (31) studied the effect of a toxic concentration of boron on prune, peach, and grape. She finds fewer chloroplasts in the leaves, and a disintegration of leaf tissue in the plants supplied with such a concentration. The smaller veinlets were often filled with a brown substance of gummy appearance. The petioles and midribs of prune often showed gum pockets resulting from a disintegration of the xylem elements. In shoots, the greatest injury was found in those that do not live beyond the first year. Sometimes phellogen developed beneath the necrotic tissues, often causing them to slough off. As in the leaf, gum pockets appeared both in the cortex and conducting system.

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A SMALL amount of zinc has been found necessarv for the normal growth of most plants. Research ha"s been stimulated during the last few years by the discovery that diseases of various crop plants are caused by a deficiency of this element. Chandler (1937), Reed and Parker (1936) and others have found that zinc deficiency may cause rosette, mottleleaf, bronzing, yellows, little-leaf or white bud, depending upon the plant affected. These can be cured by the application of zinc sulfate through the soil, into the trunk, or by a spray. Dufrenoy and Reed (Dufrenoy and Reed, 1934; Reed and Dufrenoy, 1935a, b, c; Reed and Parker, 1936; and Reed, 1938, 1939) found marked changes in cell size and structure in the leaves of affected plants. The chloroplasts were one of the first cell parts affected. There was a tendency for them to agglutinate, become vacuolated, to contain oil and verv little starch. With changes in structure changes were found in the products of metabolism.It was thought by the authors of this paper that, if deficiency symptoms were found in the roots of plants showing deficiency in the tops, a combination of the tissue culture methods developed by Bonner and Addicott (

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