In the soil, the following properties are easily recognizable as definite colloidal phenomena: water-holding capacity of clays, silts, loams, etc.; adsorption (or "fixation") of soluble plant foods so that they are not readily leached out of the soil by drainage; flocculation and deflocculation of clay, etc.
In the animal body; the contraction of muscles, the conveyance of nerve stimuli, etc., are undoubtedly accomplished by colloidal changes; and the existence of insoluble casein and fat in colloidal form in milk insures the proper nourishment of the young of nearly all species of animals.
In both plants and animals, as will be pointed out in the following chapter, practically all the vital activities of the cell protoplasm are definite manifestations of colloidal phenomena. Enzymes perform their catalytic functions by reason of their colloidal form. Proteins exist in colloidal form and are the seat of all vital functions. The regulation of the passage of materials into and out of the cell is governed by minute changes in the electrolyte concentration, etc., which produce enormous changes in the colloidal character of the protoplasm.
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.