TUBGIDITY 53 



It is in these vacuolated cells that turgidity is developed 

 to its greatest extent. It may be that continued concentra- 

 tion of the solution within the protoplasm itself may soon 

 reach a limit beyond which it cannot go without affecting 

 those energy transformations which we term vital activity or 

 life. An alteration in the activities of the protoplasm thus 

 produced may result in a change in its permeability in one 

 way or another. And changes of this sort accompanied by 

 changes in the chemical activity within the protoplast may 

 account for the formation of the vacuole and the secretion of 

 osmotically active materials into it. It was shown by Loeb 1 

 that changes in the concentration of different ions in the 

 protoplasm of animal muscle bring about marked changes in 

 its power of absorbing water. 



At any rate, however the vacuole may arise, the turgidity of 

 the normal mature plant cell is mainly due to the osmotic 

 pressure of the cell sap and to the semi-permeability of the 

 surrounding protoplasmic layer. The part played in the 

 development of turgidity by the tonoplast and ectoplast and by 

 the unmodified protoplasm itself, has not been worked out. 

 Indeed, the semi-permeability of this layer can perhaps be 

 attained only through the co-operation of the three some- 

 what distinct layers which make up the lining of the cellu- 

 lose wall. Although De Vries 2 was able to separate the 

 tonoplast from the remainder of the protoplasmic mass, yet 

 it soon lost its peculiar properties when the surrounding 

 protoplasm was killed. Pf effer 3 has shown that the tonoplast 

 and ectoplast are equivalent and are probably formed in the 



1 J. LOEB, " On Ion Proteid Compounds and Their Role in the Mechanics of Life 

 Phenomena"; I, "The Poisonous Character of a Pure NaCl Solution," Am. Jour. 

 PhysioL, Vol. Ill (1900), pp. 327-38. 



2 HUGO DE VRIES, " Plasmolytische Studien fiber die Wand der Vacuolen,' 1 

 Jahrb.f. wiss. Bot., Vol. XVI (1885), pp. 465-598. The tonoplast is not a special cell 

 organ, as De Vries was led to suppose. 



3 W. PFEFFER, " Zur Kenntniss d. Plasmahaut u. d. Vacuolen," etc., Abhandl. d. 

 k. sacks. Ges. d. Wiss. zu Leipzig, math.-physik. Klasse, Vol. XVI (1890), pp. 187-344. 



