OSMOTIC PRESSURE IN THE CELL 



139 



This needs to be emphasized with especial clearness, since the attempt has 

 frequently been made to explain a decrease of turgidity as being due to 

 an increased and more rapid power of filtration through the plasma 1 . 



The existence of a high osmotic pressure in turgid cells is made 

 evident by the extremely energetic manifestations of which they are capable. 

 These latter may be measured with fair accuracy by equilibrating them 

 against measurable forms of energy. Thus, having found by the plas- 

 molytic method the degree of concentration of an isosmotic (isotonic) saline 

 solution, the turgid force of the cell is at once given, if the osmotic pressure 

 of the isosmotic saline solution is known. 



By results obtained in this manner, it is found that the pressure of turgor 

 in land and fresh- water plants is usually equivalent to from 1-5 to 3-0 per 

 cent. KNO 3 , i.e. equals a pressure of five to eleven atmospheres 2 . Even 

 in starved and emaciated cells, the turgidity does not usually fall below one 

 per cent. KNO 3 , i. e. 3-5 atmospheres 3 . 



Frequently, the turgidity rises far above the average limits given, 

 especially in cells, where dissolved and osmotically active reserve materials 

 accumulate. Thus the pressure of turgor in the bulb of the onion (A Ilium 

 cepa), or better still the beet-root, may be equivalent, or more than equi- 

 valent, to five to six per cent, of KNO 3 (twenty-five to thirty per cent. 

 of cane-sugar), i.e. to a 'pressure of fifteen to twenty-one atmospheres 4 . 

 As the reserve materials are removed, the internal hydrostatic pressure 

 decreases, hence the turgor differs according to the stage of development, 

 and under the action of various external conditions. Since during the 

 growth and stretching of the cell- wall, turgidity remains fairly constant, 

 a regulatory and correlating adjustment must continually go on, and in 

 the same manner more or less marked differences may be maintained in 

 neighbouring cells with different osmotic and other properties 5 . 



All cells, which can accommodate themselves to concentrated nutrient 

 solutions, must be able to increase the amount of osmotic substances which 

 they contain. This is attained either by a direct absorption of the salts 

 present in the external medium (as occurs in Bacteria), or by a corre- 

 spondingly increased production of osmotically active substances. It is 

 almost entirely by the latter means that Aspergillus niger, Penicillium 



1 Pfeffer, 1890, 1. c., p. 302. 



3 For examples see de Vries, Jahrb. f. wiss. Bot, 1884, Bd. xiv, p. 427 ; Stange, Bot. Zeitung, 

 1892, p. 277 ; J. M. Janse, Permeabilitat des Protoplasmas, 1888 (Sep.-abdr. a. d. Verb. d. Akad. d. 

 Wiss. zu Amsterdam). 



8 Stange, 1. c., p. 396 ; Copeland, Einfluss von Licht u. Temperatur auf d. Turgor, 1896, p. 52. 



4 See de Vries, Sur la permeab. du protoplasma d. betteraves rouges, 1871, p. 7 (Sep.-abdr. a. 

 Archiv. Neerland., T. vi), and Unters. liber die mechanischen Ursachen d. Zellstreckung, 1877; 

 Copeland, 1. c. 



5 See Pfeffer, Druck u. Arbeitsleistungen, 1893, p. 297. The importance of these and other 

 relationships will be dealt with in the chapters directly concerning them. 



