I] INTRODUCTION 5 



once dissolve in water giving colloidal solutions. The main feature of 

 the colloidal state is that the system consists of two phases, or conditions 

 of matter. In the case of the artificial colloidal solutions first mentioned, 

 one state is solid, the gold particles; the other state is liquid, the water. 

 The solid is known as the dispersed phase, and the water as the continuous 

 phase, and such colloidal solutions are termed suspensoids. In the case 

 of proteins, starch, etc., both phases are liquid: the dispersed phase, a 

 concentrated solution of protein, etc.; the continuous phase, a dilute 

 solution of protein, etc. Such colloidal solutions are known as emulsoids. 



An important point in connexion with the colloidal state is that the 

 molecules, or aggregates of molecules, forming the dispersed phase are 

 so large that they exhibit some of the phenomena of surface energy, 

 electrical charge, etc., associated with matter in mass. These properties 

 come to be of considerable importance, when we consider how large 

 a surface is presented by matter in this state in comparison with its mass. 



A material in the plant upon which much interest naturally centres 

 is the protoplasm and the nucleus. It has been shown that the protoplasm 

 consists, chemically, largely of proteins in the colloidal state. It is itself 

 a liquid, and embedded in it are substances of various chemical constitu- 

 tion, in the form of granules of solid matter and also liquid globules. 

 Numerous chemical reactions are continually taking place in the proto- 

 plasm throughout the cell, and since many of these reactions can take 

 place both simultaneously and independently, the protoplasm must have 

 some form of organized structure. Though many phenomena of "life" 

 may be accounted for by the physical and chemical properties of such 

 substances as proteins, it is impossible to say, with our present knowledge, 

 how far all "living" phenomena may yet be explained in this way. 



Some of the main lines of metabolic S3nitheses which take place in 

 the plant will next be considered. A fundamental fact which should be 

 borne in mind is that the green plant synthesizes all the complex 

 materials of which it is composed from the simple compounds, carbon 

 dioxide, water and certain inorganic salts. The most important factor, 

 perhaps, which figures in plant metabolism, is chlorophyll. The green 

 pigments of chlorophyll are esters of complex organic acids containing 

 the elements carbon, hydrogen, oxygen, nitrogen and magnesium. They 

 have the remarkable power of absorbing the radiant energy of the sun's 

 rays and of transforming it into chemical energy, by means of which 

 carbon dioxide and water are combined to form some organic compound, 

 possibly formaldehyde, from which a simple carbohydrate is readily 

 synthesized. 



