4 INTRODUCTION [CH. 



1 they remain where they are synthesized, and build up the solid 

 ^structure of the plant, as for instance, the cell-walls. 



Matter in the colloidal state is of very great importance in the plant 

 and is probably responsible for many of the properties of "living" 

 material. Thus it will not be out of place, though it will be referred to 

 again in a later chapter, to make at this point a few remarks on the 

 colloidal state. It has been known for some time that certain metals, 

 e.g. gold and silver, and also certain metallic hydroxides and sulphides, 

 e.g. ferric hydroxide and arsenious sulphide, though insoluble in water- 

 under ordinary conditions, can, by special methods, be obtained as 

 solutions which are clear to the unaided vision. Such solutions are 

 termed colloidal. Investigation has shown that the matter is not present 

 in true solution, but in a very finely divided state, i.e. as particles many 

 times larger than simple molecules, but smaller than any particles obtain- 

 able by mechanical means of division. Such solutions are known as 

 artificial colloidal solutions, but there are a number of organic substances, 

 with very large molecules, such as proteins, starch, gums, agar, etc., which 

 at 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 its 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 



