PROGRESS IN PHYSIOLOGY 185 



reckoned with reaches the dimensions of a small library. 

 In these circumstances it is not possible to do more 

 than single out the principal lines of research and the 

 outstanding discoveries in each of them. 



ABSORPTION OF WATER AND SALTS 



Our modern views of the mode of entry of water and 

 salts in solution into root hairs date from the publication 

 of Thomas Graham's classic researches on colloids and 

 crystalloids in 1862. A few years afterwards Traube 

 applied Graham's discoveries to the elucidation of the 

 phenomena of endosmosis in plant cells and carried out a 

 series of experiments with the aid of the " artificial cell" 

 that is associated with his name. This, as you know, is 

 formed by placing a drop of a solution of gelatine in a 

 solution of tannic acid, when a colloidal film of gelatine 

 tannate is produced where the liquids meet. The gelatine 

 solution then attracts water through the tannate film, 

 diluting the gelatine within and causing the vesicle to 

 swell and finally burst, exposing a fresh surface of gelatine 

 to the tannic acid, when immediately a new membrane is 

 formed at the point of rupture. Traube found that the 

 pellicles so formed were permeable to some solutes and 

 not to others, or, in other words, were semi-permeable. 



In 1877 Pfeffer greatly extended our knowledge of 

 osmotic phenomena by the use of porous earthenware 

 pots in the walls of which semi-permeable membranes 

 had been deposited. He compared this apparatus with 

 the vegetable cell, and agreed with Traube in believing 

 that the passage of solutions through the protoplasm and 

 the cell wall depended on the relative sizes of the molecules 

 of the salts used and the spaces between the micellae of 

 the membrane. He also estimated the pressure set up 

 in such cells by the entry of various solutions and showed 

 how this pressure maintained turgidity in parenchyma. 



In the same year De Vries estimated the osmotic 



