46 PLANT PHYSIOLOGY AND ECOLOGY 



elapses before the second is set up. This is hirgely determined 

 also by the affinity for water possessed by the substances in solu- 

 tion. For example, sugar has a comparatively slight affinity for 

 water, and is much less active in osmosis than mineral salts or 

 organic acids, which show a greater attraction for water. 



59. Osmosis in root-hairs. The conditions presented by a 

 root-hair embedded in a moist soil are apparently those found 

 in ordinary osmosis. It offers a porous cellulose wall wetted 

 within by a cell sap denser than the soil water which wets it 

 throughout. A moment's reflection, however, makes it clear 

 that in such event the root-hair would sooner or later lose as 

 much water as it absorbs. The plant would then be unable to 

 get water for its various functions, especially transpiration, and 

 it would quickly wilt and die. Evidently, the root-hair must 

 be enabled to modify ordinary osmosis in such a manner that it 

 may take in more water than it gives out. This absolutely indis- 

 pensable modification of osmosis is due to the presence of the 

 protoplasmic membrane which lines the hair. This membrane, 

 like all protoplasm, has a great attraction for water. Its effect 

 is to increase the strength of the first current in osmosis (endos- 

 mose) and to decrease the return current (exosmose) . The latter 

 indeed becomes jDractically imperceptible. It is represented by 

 the slow passage of a minute quantity of acid cell sap, indicated 

 by the reaction of roots to litmus paper, or by their etching effect 

 upon a marble surface. Osmosis in the root-hair differs from 

 ordinary physical osmosis in that it consists practically of the 

 inward current alone, the return current being prevented by the 

 active properties of the protoplasm. Within the root, the nor- 

 mal process with currents in both directions takes place between 

 the cells of parenchyma. 



The osmotic properties of root-hairs are due to the fact that 

 the density of the cell-sap is normally greater than that of the 

 soil water. The substances in the hair capable of inducing os- 

 mosis are the sugars used as food, the mineral salts absorbed 

 from the soil, and the organic acids and their salts. Of these, 

 the sugars have little or no influence; the mineral salts play a 

 relatively small part, since they can be obtained only by absorp- 

 tion from without. The organic acids and salts constantly pro- 

 duced by the activities of the cell are by far the most active, 

 and it is to them that osmosis is chieflv due. 



