chap, i Processes of Absorption of Water 255 



being a considerable difference in character between the 

 external and the internal layers, which, however, he attri- 

 buted to a probable impregnation of the former by a large 

 percentage of cholesterin and lecithin. His views remain, 

 however, hypothetical. 



Pfeffer's views as to precipitation membranes, or plasma 

 membranes, were accepted by the writers of the time, but 

 more recently they met with much criticism, and by the 

 end of our period they could hardly be said to have main- 

 tained their ground in the opinion of many physiologists. 

 Chodat and Boubier, while investigating the phenomena of 

 plasmolysis in 1898, found that the protoplasmic pellicles 

 cannot be separated from the cell wall as easily as Pfeffer 

 thought. The ectoplasm appears to be adherent to the 

 young growing membrane, and when the protoplasm shrinks 

 towards the centre of the cell, as the water is removed 

 during plasmolysis, the film is actually rent or torn asunder, 

 a very thin part being recognizable adhering in places to 

 the wall. This being the case, there is no possibility of 

 an external precipitation membrane being formed between 

 the protoplasm and the wall during the ordinary process 

 of absorption. 



The failure to appreciate the fact that the cell membrane 

 of the root-hair is almost semipermeable led to incorrect 

 views as to the nature of the exudation into the soil which 

 accompanies the absorption of water and mineral salts. 

 This was held to be composed of vegetable acids, a view 

 supported by the well-known corrosive action of roots 

 upon polished marble when they come into contact with 

 its surface. It was disproved by Czapek in 1896 ; he 

 showed that with the exception of carbonic acid, no free 

 acids are excreted by the roots of terrestrial plants, but 

 that there is an exudation of acid potassium phosphate. 



Sachs' early work of 1859 and i860 on the mechanism 

 of the root-hairs was followed by further researches pub- 



