It is apparent, therefore, that the study of the colloidal condition of matter and of the properties arising out of it is of immense importance to the biochemist. No other single field is capable of yielding more fruitful results to the plant physiologist, in his studies of the response of plants to changes in their environment, or of the mechanism by which plants perform their internal functions. References BECHHOLD, H., trans. by BULLOWA, J. G. M.--"Colloids in Biology and Medicine," 463 pages, 54 figs., New York, 1919. BURTON, E. F.--"The Physical Properties of Colloidal Solutions," 200 pages, 18 figs., London, 1916. CASSUTO, L.--"Der Kolloide Zustand der Materie," 252 pages, 18 figs., Dresden and Leipzig, 1913. LIESEGANG, R. E.--"Beiträge zu einer Kolloidchemie des Lebens," 144 pages, Dresden, 1909. OSTWALD, W., trans. by FISCHER, M. H.--"Theoretical and Applied Colloid Chemistry," 218 pages, 43 figs., New York, 1911. OSTWALD, W., trans. by FISCHER, M. H.--"A Handbook of Colloid-Chemistry," 278 pages, 60 figs., Philadelphia, 1915. TAYLOR, W. W.--"The Chemistry of Colloids," 328 pages, 22 figs., New York, 1915. ZIGMONDY, R., trans. by ALEXANDER, J.--"Colloids and the Ultramicroscope," 238 pages, 2 plates, New York, 1909. ZIGMONDY, R., trans. by SPEAR, E. B.--"The Chemistry of Colloids," 274 pages, 39 figs., New York, 1917. CHAPTER XVI THE PHYSICAL CHEMISTRY OF PROTOPLASM Thus far, we have considered the chemical nature of the various groups of compounds which are found in the tissues of living organisms, laying emphasis upon those which are of plant origin. These compounds constitute the material, or machinery, of the cell, and their various transformations furnish the energy for its operation. We come now to a study of the mode of its operation, or the processes of vital phenomena.