248 The Physiology of Plants BOOKIII 



the distribution of the water with the salts it holds in 

 solution after absorption has been effected, the complicated 

 processes of exhalation of the water in view of the need 

 of a constant and copious renewal, together with the bearing 

 of these processes upon the well-being of the organism. 



To find a starting place for its solution we must go back 

 to the time of Dutrochet and recall his researches on the 

 processes of osmosis, which, though admittedly imperfect, 

 laid the foundation on which the modern views are built. 

 He laid down the fundamental proposition that concen- 

 trated solutions of various salts, enclosed in organic mem- 

 branes, apparently exercise an attraction on water in contact 

 with the other side of the membrane, so that a stream of 

 water passes through the latter to mix with the solution. 

 Such increase in volume within the membrane is capable 

 of setting up considerable hydrostatic pressure upon it. 

 His views were not in consonance with the modern concep- 

 tions of osmotic phenomena beyond a certain point, for 

 he held that there is always associated with the entry 

 of the water a flow in the opposite direction of a related 

 quantity of the salt solution, which we know now is by no 

 means necessarily the case. The theory of the excretion 

 of various acid substances by the roots, though supported 

 apparently by observations on the corrosion of marble by 

 the root-hairs, is now not at all generally held. Dutrochet's 

 results were based upon the use of very complex mem- 

 branes, which led rather to the confusion of the processes ; 

 the discovery of semi-permeable membranes came much 

 later than his time. 



Dutrochet's application of the process of endosmosis to 

 the explanation of the absorption of the mineral matters 

 of the soil, being based solely on mechanical conceptions, 

 failed to explain the variable quantities of different salts 

 which are absorbed by the same plant, or the different 

 amounts of the same salt which are taken up by different 



