278 PROTOPLASM 



tact again, thus duplicating the behavior of liquid droplets. 

 All available evidence indicates that the nuclear membrane is 

 similar in quality and behavior to the outer protoplasmic one. 



The membrane enclosing the vacuole of plant cells has been the 

 subject of much investigation, because it is presumed by some to 

 be the membrane upon which rest the osmotic properties of the 

 cell. To this membrane deVries gave the special name tonoplast. 

 Doubt has been cast on the reality of the vacuolar membrane. 

 There are those who point out that if the inner layer of streaming 

 protoplasm is carefully observed where it comes in contact with 

 the vacuolar sap, particles will be seen moving in the vacuole 

 carried along by the adjoining flowing protoplasm. The observa- 

 tion suggests that if the surface of the protoplasm, where it 

 touches the vacuole, is flowing, as it must be to move particles in 

 the vacuole, then there can be no true membrane. The weakness 

 in this argument lies in the so often met with failure to appreciate 

 that the protoplasmic membrane is itself living protoplasm and may 



undergo the same changes in 

 activity, in particular viscosity, 

 as does the inner protoplasm 

 , , _. ^ which it bounds. 



^ " c V j^ ^g possible, with the aid of 



Fig. 142. — The vacuole («) of a (now . . , , 



dead) plant cell separated by a needle miCrO-needleS, tO Withdraw an 



(p) from the cytoplasm (c) and nucleus entire (plasmolyzed) protoplast 



(n) and floating apart as a sack. :^ + „„+ 



^ ' (the cell contents as an intact 



entity) from the cellulose enclosure of a plant cell (Fig. 142). 

 Once so isolated, the protoplasm coagulates; it may then be 

 stripped off from the vacuole, leaving the latter freely suspended, 

 floating like a water-filled sac; this is only possible because the 

 aqueous vacuolar sap is enclosed by a membrane. Any part of 

 such a dismembered cell is, of course, not normal, but J. Plowe 

 has isolated living intact cells in a similar way and found evidence 

 from miscrodissection that the tonoplast — the vacuolar mem- 

 brane — is a distinct morphological structure. That the proto- 

 plasm is still alive is mdicated by a continuation of streaming. 

 The Austrian plant physiologist Karl Hofler, in similar experi- 

 ments, shows the tonoplast to be fluid, as indicated by streaming, 

 but to possess, though fluid, considerable resistance and resilience, 

 just as the liquid membrane of a soap bubble is resistant to 

 pressure. Hofler also regards the tonoplast as functioning some- 



