252 The Physiology of Plants BOOK in 



of the young roots. He went on to show how the water 

 enters the root-hair by the process of osmosis, carrying with 

 it traces of the salts present in the particles of the soil. 



Sachs further determined the amounts of water which 

 different kinds of soil can retain with advantage to the 

 plants growing in them, and their content when saturated 

 with it ; he showed that saturation is unnecessary by 

 ascertaining how much the soil contains at the moment 

 when the plants growing in it commence to wither, proving 

 thereby that the quantities available for the use of such 

 plants fall far below those which the soil can contain. 

 Incidentally he showed that the available water in a clay 

 soil is to that in a sandy one in the proportion of 44 : 19. 



The osmotic processes received subsequently little de- 

 tailed attention till 1877, research during the interval being 

 directed chiefly to the problems connected with the method 

 of conduction of the stream from the roots throughout the 

 plant. In that year appeared two important contributions 

 to our knowledge of the part played by osmosis generally 

 in the movements of liquids in vegetable tissues. 



The most noteworthy of these was Pfeffer's Osmotische 

 Untersmhungen, which really forms the foundation of the 

 views of the role of osmosis in the vegetable cell held up 

 to the end of the century. The researches on which it is 

 based paved the way to Van 't Hoff's theory of osmotic 

 pressure, advanced by him many years later. Pfeffer 

 directed his attention particularly to the character of the 

 membranes and for the first time pointed out the importance 

 of the part played by the protoplasm. He claimed that 

 the latter, lying as a thin film inside the cell wall, is not 

 homogeneous, but that its surfaces, the one abutting on 

 the wall and the other on the vacuole, are permeable with 

 greater difficulty than the softer intermediate substance. 

 This difference of permeability he attributed to what he 

 called ' plasma membranes ', which he held to be composed 



