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.