SECTION VII. 



COLLOIDS. 



The translocation of dissolved substances from cell to cell in 

 the living plant is largely dependent on the dififusibility of 

 liquids through colloidal membranes, and since some of the 

 fluids occurring in plants are in reality colloidal solutions, it 

 is desirable to consider briefly some of the more important 

 properties of such solutions. 



While studying the phenomenon of diffusion, Graham, in 

 1 86 1, found that not all substances behaved in the same way 

 when separated from water by a membrane of parchment or 

 some similar material ; thus, while such a substance as cane 

 sugar passed through the membrane readily, albumen did not. 



Crystallizable substances which, like cane sugar, were able 

 to diffuse, he termed crystalloids, whilst those substances which, 

 like starch, albumen, and gums, were not crystalline, and would 

 not diffuse, he called colloids. 



This difference may be illustrated by placing a mixture of 

 starch paste and a solution of common salt in a parchment 

 dialyser,* and floating this upon a large volume of distilled 

 water. If a small portion of the mixture, and of the surround- 

 ing water, be tested from time to time with silver nitrate and 

 with iodine solution, it will be found that the salt passes 

 through the membrane, but the starch does not. 



Graham further found that it was possible, under certain 

 conditions, to cause otherwise insoluble substances to form 

 colloidal solutions. Thus by adding an excess of dilute hydro- 

 chloric acid to a dilute solution of sodium silicate he obtained 

 a clear solution instead of a precipitate of silicic acid. On 

 subjecting this solution to dialysis, the sodium chloride was 

 washed out, and there remained behind a clear liquid contain- 



* The dialyser may consist of a parchment bag, or of a tray formed by stretch- 

 ing a sheet of parchment over a circular wooden frame. 



273 18 



