viii Contents 



CHAP. PAGE 



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 cul- 



tures 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 cul- 



tures 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 i, . . . 62 



2. Fungi 63 



Conclusion 64 



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 



