OSMOTIC PRESSURE IN THE CELL 137 



smaller and ultimately disappear as this substance diosmoses away. In 

 the same way the gas of gas-vacuoles must be under compression 1 . 



From what has been said, it follows that in plasmodia or other 'soft 

 gymnoplasts, no highly osmotic substances can be present in solution. 

 Although the presence of highly osmotic solutions is, hence, not an 

 essential feature of the protoplasmic constitution, nevertheless the possibility 

 remains, that in other cases marked osmotic forces may be developed in the 

 plasma, in correspondence with the fact that the osmotic powers of the cell- 

 sap may be only trifling or very strong, according to the ends which are 

 to be attained and in adaptation to the surrounding relationships 2 . It is 

 always possible, for example, that in the plasma of Myxomycetes con- 

 centrated solutions of feebly osmotic substances may be present. Except 

 for the maintenance of vacuoles, the existence of marked osmotic pressure is 

 not in general essential, although in dermatoplasts turgidity is an essential 

 condition for normal growth, and hence for the maintenance of the life of 

 the organism. Turgidity also serves to make soft tissues firm and rigid, and 

 thus enables work to be done against considerable external resistance. 



The internal hydrostatic pressure will be highest when the plant is 

 lying in water, and when the cell-walls are permeated with pure water, 

 for if they are imbibed with a saline solution, the pressure of turgor will 

 be decreased by the corresponding osmotic equivalent, while, when the 

 solution is sufficiently concentrated, water is withdrawn from the protoplast 

 until plasmolysis occurs. A solution which is just able to remove the 

 turgidity of the cell is one which has the same osmotic value as the cell-sap 

 has. To produce the same effect by solutions of various substances, these 

 must have the same osmotic value, and hence by observing the degree 

 of concentration of different solutions necessary to produce incipient 

 plasmolysis, the osmotic value of the different substances used may be 

 estimated. This method does not determine the absolute osmotic pressure, 

 but the relative values thus obtained will enable such determinations to be 

 made for all the substances under observation as soon as the absolute value 

 is determined for any one of them. 



For non-diosmosing substances, the osmotic pressure is independent 

 of the nature of the semi-permeable membrane. Hence physical measure- 

 ments made by artificial means give accurate values for the pressure 

 exerted when in contact with the plasmatic membrane. According to such 

 physical estimations a one per cent, (volume) solution of cane-sugar at I5C. 

 develops a pressure of 0-69 atmospheres, a one per cent, solution of KNO 3 , 3-5, 

 and a similar solution of NaCl, 4-16 atmospheres (Table, p. 146). 



1 Klebahn, Flora, 1895, p. 241 (for Phycochroinaceae) ; Engelmann, Zool. Anzeiger, 1878, p. 152, 

 and in Hermann's Handb. d. Physiol., 1879, Bd. I, p. 348. 



2 Turgor and osmotic pressure are not necessarily dependent upon the existence of vacuoles, as 

 Went (Jahrbuch f. wiss. Bot., 1888, Bd. XIX, p. 350), for example, erroneously assumes. 



