VI. EFFECT OF BORON COMPOUNDS 65 I. Presence of Boron in Plants 65 II. Effect of Boron on the Growth of Higher Plants 67 1. Toxic effect 67 (a) Toxic action of boron compounds in water cultures 67 (b) Toxic action of boron compounds in sand cultures 70 (c) Toxic action of boron compounds in soil experiments 71 2. Effect of boron compounds on germination 72 3. Does boron stimulate higher plants? 73 (a) Water cultures 73 (b) Sand cultures 73 (c) Soil cultures 74 III. Effect of Boron Compounds on Certain of the Lower Plants 76 Conclusion 77 VII. EFFECT OF MANGANESE COMPOUNDS 78 I. Presence of Manganese in Plants 78 II. Effect of Manganese on the Growth of Higher Plants 81 1. Toxic effect 81 (a) Toxic action of manganese compounds in the presence of soluble nutrients 81 (b) Toxic action of manganese compounds in sand cultures 82 (c) Toxic action of manganese compounds in soil cultures 82 2. Effect of manganese compounds on germination 84 3. Does manganese stimulate higher plants? 84 (a) Stimulation in water cultures 85 (b) Stimulation in soil cultures 86 III. Effect of Manganese Compounds on Certain of the Lower Plants 90 IV. Physiological Considerations of Manganese Stimulation 90 Conclusion 92 VIII. CONCLUSIONS 93 BIBLIOGRAPHY 97 INDEX OF PLANT-NAMES 107 GENERAL INDEX 109 LIST OF ILLUSTRATIONS x LIST OF ILLUSTRATIONS FIG. PAGE 1. Sketch illustrating water culture methods 12