102 The Structure of Protoplasm 



left intact when the rest of the cytoplasm has become disorganized 

 and separated from it. The isolated tonoplast persists as a fluid 

 semipermeable sac enclosing the vacuolar sap, and may preserve 

 its integrity for days or even weeks. By contrast, the external 

 surface film has never been completely separated from the proto- 

 plast, but probably this is due simply to its position, as local 

 elevation sometimes occurs naturally and may be produced artifi- 

 cially. Both surfaces of the tonoplast resemble the surface of the 

 ectoplast in showing immediate adhesion to oil but adhesion to 

 glass only after mechanical disturbance. An oil drop which has 

 snapped onto the outer surface of a freed tonoplast is deformed 

 somewhat by stretching of the latter but not to the same extent as 

 at the cell surface. It is possible that this slight display of elastic 

 property may be due to an adsorbed film of protein from the endo- 

 plasm. Chambers and Hofler emphasize the highly fluid, though 

 cohesive and extensible, nature of the tonoplast and the immisci- 

 bility of its substance with water. 



On the whole the physical properties of the tonoplast and ecto- 

 plast seem to be very similar, and Plowe has described how the 

 two films may merge to form a single envelope. 



IS THERE AN INNER GEL LAYER? 



Since the cytoplasm of the plant cell is bounded on its vacuolar 

 side by a fluid layer similar to the surface film, the question arises 

 whether there is also an inner zone which corresponds to the cor- 

 tical plasmagel. The frequent elasticity of transvacuolar strands 

 and of strands pulled by micro-needles into the vacuole, as com- 

 pared with the relatively inelastic quality of strands pulled from 

 an isolated tonoplast, suggests that the elasticity of the former 

 resides in the core of endoplasm which such strands usually con- 

 tain. On the other hand, an oil drop applied to the surface of the 

 vacuole sinks into the cytoplasm much more easily than a similar 

 drop applied to the outside, which indicates that if an inner gel 

 layer exists it is much less substantial than the outer one. 



OTHER CYTOPLASMIC FILMS OR MEMBRANES 



It seems reasonable to regard the colorless film which covers 

 chloroplasts and can be elevated from them in pathological swell- 

 ing as being equally with the ectoplast and tonoplast a cytoplasmic 

 structure. Even the nuclear membrane, or part of it at least (since 



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