THE COLLOIDAL CONDITION 219 



deposition; the purification of sewage; the manufacture of soaps; 

 the manufacture of butter, cheese, and ice cream; fruit jellies, 

 salad dressings, etc. This list could be extended to great length, 

 but is already long enough to emphasize the very great importance 

 and practical value of colloidal ohenomena in daily life. 



NATURAL COLLOIDAL PHENOMENA 



Many of the phenomena of nature are colloidal in character. 

 These may be observed in the mineral, the animal, and the 

 vegetable kingdoms. Here, again, a lengthy discussion of the nature 

 of these phenomena would be out of place in this connection, and a 

 few typical examples will serve to illustrate the general importance 

 in nature of this property of matter. 



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 convey- 

 ance of nerve stimuli, etc., are undoubtedly accomplished by col- 

 loidal 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 fol- 

 lowing chapter, practically all the vital activities of the cell pro- 

 toplasm are definite manifestations of colloidal phenomena. 

 Enzymes perform their catalytic functions by reason of their col- 

 loidal 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 elec- 

 trolyte concentration, etc., which produce enormous changes in 

 the colloidal character of the protoplasm. 



It is apparent, therefore, that the study of the colloidal con- 

 dition 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 func- 

 tions. 



