256 The Physiology of Plants book hi 



lished in 1865. These showed how the copious absorption 

 of water and its osmotic transference to the cells of the 

 cortex of the root lead to the setting up of a very consider- 

 able turgescence in this region, the water passing from cell to 

 cell under the same forces as led to its original absorption 

 from the soil. The turgescence in its turn is accompanied 

 by a very considerable hydrostatic pressure, which is 

 brought to bear upon the cell membranes, and which, not 

 being resisted by any fluid in the vessels of the wood, brings 

 about a filtration into them. This enabled him to associate 

 the processes of absorption with the phenomena of bleeding 

 from cut stems, which had much earlier been observed and 

 to some extent investigated by Hales. To establish this 

 theory of filtration under pressure, Sachs found it necessary 

 to postulate a greater permeability of the membranes on 

 the sides of the cells directed towards the centre of the 

 axis than on the rest of their surface, in order to show 

 how the direction of the stream is secured. Though this 

 has never been disproved, it has never, on the other hand, 

 been satisfactorily established, and it is rather difficult to 

 see why the pressure does not force the water from the 

 cells into the intercellular spaces throughout the cortex, 

 as they are distributed symmetrically through the tissue. 

 Sachs' presentation of the theory is worthy of note, for 

 though he originally illustrated it by a merely mechanical 

 model, with membranous walls of different thicknesses on 

 the two sides of the artificial cell used, he pointed out that 

 the resistance to the filtration is a matter in which the 

 protoplasmic utricle is largely concerned, a view which has 

 come more and more into prominence since his work 

 appeared. 



Sachs named the pressure so set up in the cortex, in 

 consequence of the over-distension of the cells, root-pressure, 

 and attributed to it the initiation of the stream of the 

 transpiration current, or ascending sap. A little later he 



