IV. EFFECT OF ZINC COMPOUNDS 36 I. Presence of Zinc in Plants 36 II. Effect of Zinc on the Growth of Higher Plants 38 1. Toxic effect 38 (a) Toxic action of zinc salts alone in water cultures 38 (b) Effect of soluble zinc salts in the presence of nutrients 39 (c) Effect of zinc compounds on plant growth when they are present in soils 41 (d) Mode of action of zinc on plants 43 2. Effect of zinc compounds on germination 43 3. Stimulation induced by zinc compounds 45 (a) Stimulation in water cultures 45 (b) Stimulation in sand cultures 46 (c) Increased growth in soil 46 4. Direct action of zinc salts on leaves 47 III. Effect of Zinc on Certain of the Lower Plants 48 Conclusion 50 V. EFFECT OF ARSENIC COMPOUNDS 51 I. Presence of Arsenic in Plants 51 II. Effect of Arsenic on the Growth of Higher Plants 52 1. Toxic effect 52 (a) Toxic action of arsenic compounds in water cultures in the presence of nutrients 52 (b) Toxic effect of arsenic compounds in sand cultures 57 (c) Toxic effect of arsenic when applied to soil cultures 57 (d) Physiological considerations 59 2. Effect of arsenic compounds on germination 60 3. Do arsenic compounds stimulate higher plants? 61 III. Effect of Arsenic Compounds on Certain of the Lower Plants 62 1. Algae 62 2. Fungi 63 Conclusion 64