Structural Differentiation of Cytoplasm 101 



this can be removed without harm to the cell. The hyaline film 

 on amoeba is also fluid and cohesive. According to Mast the fluid 

 film is here bounded by a more rigid pellicle, the plasmalemma. 



In the case of plant cells a so-called hyaline layer on the surface 

 becomes visibly apparent only in very fine strands pulled out from 

 the surface. Incidentally, though it has been the custom to call the 

 film hyaline, it is by no means free from granules. But the very 

 fact that strands of fluid consistency can be pulled out from the 

 surface of a cell which has been dehydrated by plasmolysis until its 

 gel layer is rigid and brittle, is evidence for the existence of a differ- 

 entiated liquid layer overlying the gel. The fine strands which 

 connect a plasmolysed plant protoplast with the cell wall, being 

 composed entirely of the film substance, reveal some of its proper- 

 ties by their behavior. In strong plasmolytes they stiffen suffi- 

 ciently to crumple up on breaking, but tenuous as they are, this 

 stiffening seems to be confined to a still thinner surface film, since 

 under the same conditions of dehydration they become beaded on 

 standing due apparently to the running into drops of a more liquid 

 core. All this indicates that the protoplasmic surface layer may have 

 a denser film of its own — compare the plasmalemma of amoeba. In 

 spite of this capacity for hardening at its surface, the ectoplast 

 shows little sign of elastic extensibiUty. That quality of the proto- 

 plasm seems to reside mainly in the layer beneath. 



Some properties of the surface film in plant cells are brought 

 out by the behavior of applied oil drops. On making contact with 

 the protoplasmic surface, a body of oil, even when large, relative 

 to the cell, immediately has its surface tension greatly reduced so 

 that it will often maintain shapes of nonminimal area. That the 

 surface-active material spreads as a film over the oil is shown by the 

 speed with which the change occurs and by the dragging of gran- 

 ules and chloroplasts with it, especially when the oil drop is large. 

 Further, the transport of these bodies through the cytoplasm to the 

 drop seems to prove bodily movement of a film over the proto- 

 plasm. The movement of the film is not prevented by plasmolysis 

 which is strong enough to make the gel layer quite stiff, which is 

 further proof that the surface film is distinct from the gel layer. 



THE TONOPLAST 



At the inner cytoplasmic surface in contact with the central 

 vacuole is another film, the tonoplast of de Vries, which is often 



