2. Photograph. Barley grown with copper sulphate _To face_ 20 3. Curve. Ditto 21 4. Photograph. Peas grown with copper sulphate _To face_ 29 5. Curve. Ditto 29 6. Curve. Barley grown with zinc sulphate 40 7. Photograph. Peas grown with zinc sulphate _To face_ 40 8. Curve. Ditto 41 9. Photograph. Barley grown with arsenious acid _To face_ 54 10. Curve. Ditto 55 11. Curve. Peas grown with arsenious acid 55 12. Curve. Barley grown with arsenic acid 56 13. Curve. Barley grown with sodium arsenite 56 14. Curve. Peas grown with sodium arsenite 57 15. Curve. Barley grown with boric acid 69 16. Photograph. Peas grown with boric acid _To face_ 69 17. Curve. Barley grown with manganese sulphate 85 18. Photograph. Ditto _To face_ 86 19. Photograph. Peas grown with manganese sulphate 86 CHAPTER I INTRODUCTION Ever since the physiological side of botany began to emerge from obscurity, the question of the relation between the nutrition and the growth of the plant has occupied a foremost position. All kinds of theories, both probable and improbable, have been held as to the way in which plants obtain the various components of their foods. But quite early in the history of the subject it was acknowledged that the soil was the source of the mineral constituents of the plant food, and that the roots were the organs by which they were received into the plant. A new chapter in the history of science was begun when Liebig in 1840 first discussed the importance of inorganic or mineral substances in plant nutrition. This discussion led to a vast amount of work dealing with the problem of nutrition from many points of view, and the general result has been the sorting out of the elements into three groups, nutritive, indifferent, and toxic. Thus calcium, phosphorus, nitrogen and potassium are classed as nutritive, arsenic, copper and boron as toxic, and many others are regarded as indifferent.