136 THE MECHANISM OF ABSORPTION AND TRANSLOCATION 



exerted upon it '. Though such an arrangement appears to be advan- 

 tageous, the facts at our disposal do not suffice to form a definite opinion. 

 Very concentrated solutions of albuminous or other colloid substances may 

 be present in the plasma, but their osmotic energy is extremely feeble. 

 Colloid substances do actually exhibit transitional stages between the swollen 

 and the dissolved condition (Chap. III). However that may be, the semi- 

 fluid nature of the plasma renders possible rapid adjustment to differences 

 of pressure. This is directly shown by plasmolysis, in which phenomenon, 

 provided the cell-wall allows the saline solution to pass readily through, 

 the plasma contracts until a condition of equilibrium is reached (Fig. 10), 

 the cell-sap giving up water until its osmotic concentration is equal to that 

 of the fluid outside. The osmotic pressure is proportional to the degree 

 of concentration, but the force of imbibition increases in geometric pro- 

 portion as the amount of water present decreases (Sect. 12). Hence, if 

 the respective decreases in the volume of the plasma and cell-sap were 

 known, it would be possible to determine whether it is the osmotic energy 

 of the plasma, or its power of swelling, which resists the pressure exerted 

 upon it. Every vacuole must alter its size, when necessary, until its contents 

 become isosmotic with the rest of the cell, and hence it is possible by 

 observing these changes of volume to determine the precise spot where an 

 increased or decreased hydrostatic pressure originates 2 . 



The semi-fluid plasma can interpose no appreciable hindrance to the 

 full transmission of the internal osmotic pressure to the cell-wall outside. 

 Nor can the force of turgor be perceptibly raised above the osmotic 

 pressure of the cell-sap by any active contraction of the plasma, while 

 owing to the loose arrangement of its component parts, the tendency of 

 the stretched plasma to passive contraction cannot possibly exercise any 

 appreciable effect in increasing the internal hydrostatic pressure. 



The pressure of the cell-sap against the plasma and cell-wall will be 

 partially antagonized, according to definite physical laws, by the centrally 

 directed pressure which every curved limiting membrane exerts. Since 

 the tangential pressure existing in the peripheral membrane is not precisely 

 known, this centripetal force cannot be exactly determined. Nevertheless 

 in cells of ordinary size it is of trifling importance in comparison to the 

 osmotic pressure, for even with a radius of o-oi mm. it cannot exceed 0-03 

 to 0-06 atmospheres. In vacuoles of 0-0005 mm. radius the centripetal 

 pressure would, it is true, reach 1-5 to 3 atmospheres, and hence without 

 a correspondingly increased internal pressure such vacuoles could not be 

 maintained or even formed. Similarly the vacuoles which may be artificially 

 produced in a plasmodium by means of asparagin, gradually become 



1 Pfeffer, Osmot. Unters., 1877, p. 170; Plasmahaut u. Vacuolen, 1890, p. 294. 



2 Pfeffer, Osmot. Unters., 1877, p. 180 seq. ; Plasmahaut u. Vacuolen, 1890, pp. 296, 322. 



