250 The Physiology of Plants BOOK in 



various directions, so that it has only gradually become 

 possible to give a co-ordinated account of them. Even 

 now many difficulties remain unexplained and open out the 

 way to further researches, which are still in progress. 



The classic investigations of Graham into the phenomena 

 of osmosis were published in 1862, and were very quickly 

 taken advantage of by the leading physiologists of the time. 

 The work undertaken by Traube was the first definite 

 application of Graham's researches to the mechanics of the 

 vegetable cell, and particularly to its mode of growth and 

 the part played therein by endosmosis. His work, which 

 appeared in 1867 and in 1874, dealt at some length with the 

 structure of the cell wall, which he demonstrated to be very 

 largely colloidal. Starting from Graham's results, Traube 

 prepared artificial cells in which the conditions of the 

 ordinary vegetable cell were reproduced with approximate 

 accuracy. His preparations were made by placing a drop 

 of syrupy solution of a specially prepared gelatine into 

 a solution of tannic acid, when a precipitate of tannate 

 of gelatine immediately formed around it. He obtained 

 thus a colloidal membrane surrounding a solution of a 

 colloid. The osmotic attraction of the gelatine continually 

 drew water through the membrane into the space enclosed 

 in the pellicle of gelatine tannate, setting up hydrostatic 

 tension and continuously diluting the solution of the gela- 

 tine inside. Traube held that a pellicle prepared in that way 

 corresponded fairly closely in its behaviour with that of an 

 ordinary cell wall, particularly if a little lead acetate or 

 copper sulphate was added to the gelatine. His experi- 

 ments led him to support Naegeli's theory of the formation 

 of cell wall by the process of intussusception. 



Experiments made with pellicles produced by the reaction 

 of colloidal with crystalloidal substances showed them to 

 be impermeable to" the fluids from which they were pro- 

 duced, but to allow certain other chemical compounds to 



