Chapter XV

) of the protoplasm of the plant cells in such a way as to destroy its permeability to nutrient substances, while mixtures of salts restore the proper state of colloidal dispersion and permit the normal functioning of the protoplasm.

It is apparent from the above brief discussions that the rôle of the different soil elements as plant food, and their relations to the complex processes which constitute plant growth, afford an interesting and promising field for further study.

References

BRENCHLEY, WINIFRED E.--"Inorganic Plant Poisons and Stimulants," 106 pages, 18 figs., Cambridge, 1914.

HALL, A. D.--"Fertilizers and Manures," 384 pages, 7 plates, London, 1909.

HALL, A. D.--"The Book of the Rothamsted Experiments," 294 pages, 49 figs., 8 plates, London, 1905.

HOPKINS, C. G.--"Soil Fertility and Permanent Agriculture," 653 pages, Chicago, 1910.

HILGARD, E. W.--"Soils," 593 pages, 89 figs., New York, 1906.

LOEW, O.--"The Physiological Rôle of Mineral Nutrients," U. S. Department of Agriculture, Bureau of Plant Industry, _Bulletin_ No. 45, 70 pages, Washington, D. C., 1903.

RUSSELL, E. J.--"Soil Conditions and Plant Growth," 243 pages, 13 figs., _Monographs_ on Biochemistry, London, 1917. (3d ed.)

WHITNEY, M.--"A Study of Crop Yields and Soil Composition in Relation to Soil Productivity," U. S. Department of Agriculture, Bureau of Soils, _Bulletin_ No. 57, 127 pages, 24 figs., Washington, D. C., 1909.

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