TUEGIDITY 55 



pressure of the solutes outside, together with the slight 

 resilience of the protoplasmic layer, cause a contraction of 

 the protoplasm resulting in its separation from the inclosing 

 cellulose ■wall. If the process of plasmolysis is complete the 

 vacuole may disappear, practically all the water passing out. 

 In such cases the protoplasm often takes on the form of a 

 solid sphere, which lies near the middle of the cell or at one 

 side. Plasmolysis comes about within a very few minutes 

 after the cell has been placed in the plasmolyzing solution. 

 It is partly from the latter fact that the cellulose wall is 

 known to be permeable to solutes as well as to water. If it were 

 not so, either plasmolysis would not occur, or the cellulose 

 membrane would follow the protoplasm in its withdrawal 

 toward the center of the cell. The cellulose wall does, 

 indeed, contract to a certain measurable extent, but this is 

 due entirely to its elasticity; it simply returns to its normal 

 state of equilibrium when the internal pressure of the turgid 

 protoplasmic sac is removed. 



This fact of plasmolysis has long been known,' but the 

 true interpretation of it was due to De Vries ^ and Pf effer.' 

 After the relation which exists between plasmolysis, turgidity, 

 and osmotic pressure was once established, it was De Vries* 

 who pointed out that in the former of these phenomena we 

 possess a means of measuring the amount of osmotic pressure 

 in any given cell. His method has been used very largely 

 in such measurements. It may be described as follows: 

 If a piece of plant tissue be placed in a concentrated solution 



1 For a historical treatment of this subject see W. Pfeffee, " Zur Kenntniss 

 der Plasmahaut u. d. Vacuolen," eto., AbTiandl. d. k. sOchs. Ges. d. Wins, zu Leipzig, 

 math.-physik. Klasse, Vol. XVI (1890), p. 316. 



2H. DeVeies, Untersuchungen ilber die mechanischen Uraachen der ZelUtrech- 

 ung, Leipzig, 1877. 



8 W. Pfeffee, OsmoUsche Untersuchungen, Leipzig, 1877, pp. 121 ff. 



* H. De Vkies, " EIne Methode zur Analyse der Turgorkraft," Jahrb. f. wisa. Bot, 

 Vol. XIV (1884), pp. 427-601. ipem, "Osmotische Versuohe mit lebeuden Membranen," 

 Zeitschr.f.physik. Chem., Vol. II (1888), pp. 415-32; IDEM, "Isotonische Koefiizienten 

 einiger Salze," ibid.. Vol. Ill (1889), pp. 103-12. 



