OSMOTIC PRESSURE IN THE CELL 143 



the difference of potential between the fluids inside and outside the cell is 

 equilibrated. Rapid variations of turgidity may be produced in the same 

 manner, and these probably often occur, but can be detected only when 

 they induce reactions which are themselves visible, as for example when the 

 anther-filaments of Cynareae shorten on stimulation, owing to a sudden 

 diminution of turgidity allowing the stretched and elastic cellulose mem- 

 branes to contract. To permit a rapid reaction, a rapid filtration of water 

 out of the cells is all that is required. If this is assured, then under par- 

 ticular conditions the same result must always be produced, independently 

 of whether the fall of osmotic pressure is due to the removal of a salt, or 

 to a chemical change, or to some other means. Various chemical changes 

 are possible, which, without any precipitation being produced, may lead to 

 a sudden diminution of the osmotic pressure, while a power of returning 

 to the original condition of equilibrium is in every case possible and indeed 

 necessary l . 



A reduction of turgidity due to the vital activity of the plant itself 

 must finally lead to a contraction of the protoplast, similar to that caused 

 by plasmolysis, and produced in the same way as when a diminution in the 

 amount of substance dissolved in a vacuole causes it to become smaller. 

 Examples of such contractions are afforded during the process of con- 

 jugation in Spirogyra, and also when cell-formation is accompanied by 

 rejuvenescence. 



Osmotic energy, it must be clearly borne in mind, is simply the 

 mechanical means employed by the plant to attain various ends, and made use 

 of in developmental processes, as well as in various reactions to stimulation. 

 Any change taking place in the osmotic properties of either the cell-sap 

 or plasma must cause a corresponding alteration in the tension of the 

 stretched cell-wall and be followed by a readjustment of the entire osmotic 

 system. 



Historical. Nageli was not only the first to recognize the extreme importance 

 of the diosmotic peculiarities of the cell, but he also clearly understood the causes of 

 turgidity, and recognized that further plasmolysis ceases as soon as the external fluid 

 is isosmotic with the cell-sap 2 . De Vries first used the term ' plasmolysis,' and the 

 unexpectedly high values, which osmotic pressure was found capable of attaining, 

 were shown by Pfeffer to be the natural consequences of the physical agencies at 

 work 3 . By de Vries the relative osmotic values of different substances were 



1 See Pfeffer, Plasmahaut u. Vacuolen, 1890, p. 320, and Studien z. Energetik, 1892. 



2 Nageli, Pflanzenphysiol. Unters., 1855, I, p. 21. For details see Pfeffer, Plasmahaut u. 

 Vacuolen, 1890, p. 316. Here it is also mentioned that Dutrochet was the first to use saline solu- 

 tions to remove turgor. 



3 Pfeffer, Period. Beweg., 1875, p. in ; Physiol. Unters., 1873, p. 119; de Vries, Unters. iiber 

 d. median. Ursachen d. Zellstreckung, 1877; Pfeffer, Osmot. Unters., 1877. 



