THE VITAL PROPERTIES OF THE CELL 141 



and so produces a current, the result being that the bladder swells 

 up in consequence of the increased pressure of its contents, and its 

 wall grows thinner and thinner. The distension of the bladder 

 only ceases when the external and internal liquids are in osmotic 

 equilibrium. Thus the protoplasmic coating of many plant-cells 

 would be very much distended in consequence of the internal 

 pressure (turgor) were it not that a limit is set to its distension 

 by the less elastic cellulose membrane. 



Equilibrium between the cell-sap and the surrounding fluid 

 mio-ht be established, if the osmotic substances were to become 

 diffused into the water, so as to remove the cause of the internal 

 pressure. However, this is prevented by the properties of the 

 living plasmic membrane. As the plasmic membrane, if the ex- 

 pression may be allowed, decides whether a body may be admitted 

 into the interior of the cell or no, similarly it has the important 

 power of retaining in the cell-sap dissolved substances which 

 otherwise would be washed out by the water bathing the cell ; of 

 this property mention has already been made, and an instance 

 cited (Pfeffer V. 23). 



That, in fact, the cell-sap exists under a pressure greater than 

 that of its environment, for instance, that the pressure in aquatic 

 plants is greater than that of the surrounding water, may be 

 easily proved by some simple experiments, as has been shown by 

 Nageli (V. 16). If a cell of Spirogyra be opened by an incision, 

 so that part of its contents flows out, the transverse walls of the 

 two neighbouring cells bulge out towards the cavity of the injured 

 one. Hence the pressure in the uninjured cells must be greater 

 than that in the injured one, the tension of which has sunk down 

 to the level of that of the surrounding water. 



3. Absorption of Solid Bodies. Cells, which either are not 

 surrounded by a special membrane, or possess apertures in 

 their membranes, are able to take solid bodies up into their 

 protoplasm, and to digest them. Thus Rhizopoda capture other 

 small unicellular organisms with which their widely outstretched 

 pseudopodia come into contact (Figs. 10, 60). The pseudopodia 

 which have seized the foreign body contract, and so gradually 

 draw it into the mass of the protoplasm ; here the nutrient sub- 

 stances are extracted, whilst the indigestible remains, such as 

 skeletal structures, are after a time ejected to the exterior. Even 

 solid substances, which possess but small nutritive value, are taken 

 up. If carmine or cinnabar granules are introduced into the water, 



