Conditions and Principles of Absorption 73 



cose, organic acids, and many other compounds are also 

 important in the osmotic strength of cells. The high 

 turgor of certain mold-fungi growing upon concentrated 

 solutions has been determined to be due to organic sub- 

 stances, which may be readily produced within the cell. 



46. Osmosis and the absorption of nutrient salts. 

 The water requirement is not the only one with which 

 osmosis is concerned, for the principles of osmosis and 

 diffusion govern also the absorption of nutrient salts; 

 likewise, of course, the absorption of any other substances 

 present in the soil solution. Moreover, the plasmatic 

 membrane is to a degree permeable to all the nutrients, 

 and to many other substances as well. 



Each substance in the soil solution has its specific tend- 

 ency to diffuse, and it therefore tends to come to equilib- 

 rium with the tension of the same substance in the cell. 

 The cells which are active in absorption have in turn a 

 relation to those adjacent to them, and this relation, 

 emphasized or otherwise modified by cells especially ca- 

 pacitated for conduction, extends to all parts of the com- 

 plex organism. The root-hair, then, in so far as it is 

 permeable, absorbs each substance or solute particle inde- 

 pendently, and in accordance with a certain attraction for, 

 or use of, that substance in some way, as in the deposition 

 in an insoluble form it may be in the building up of pro- 

 toplasm, or in the accumulation of complex food-materials. 



One of the most remarkable facts respecting the osmotic 

 relation of the plant to the soil solution is that there is so 

 little exosmosis, or outward diffusion of substances from 

 the plant, substances present in the plant but not in 

 the soil. Again, it is difficult to understand the absorp- 



