636 R. J. GOLDACRE 



(c) INERT BAND IN MID-REGION OF RHIZOID CELL (PIG. I, d) 



More seldom, a sudden increase in cytoplasm thickness, accompanied 

 by wrinkling of tonoplast and vacuole formation, can be observed in the 

 mid-region of rhizoid cells. Close inspection indicates that the tangential 

 force on the cytoplasm here ceases to operate — cytoplasmic particles 



O 



^ 



D 





Fig. I. Diagram showing regions of tonoplast contraction in Nitella. A, tip of 

 rhizoid cell; B. elongated internodal cell, at junction with neighbouring cells 

 (chloroplasts are omitted for clarity); C, junction of two cells in rhizoid; D, active 

 patch in mid-region of rhizoid. Note formation of small vacuoles within some 

 wrinkles (C, D). These subsequently unite with the central vacuole. 



which elsewhere move fastest near the cell membrane here move slowest 

 near the cell membrane, so that the cytoplasm rides up over itself and 

 forms wrinkles. 



4. Discussion 



Close inspection of the wrinkles indicated that their area decreased 

 rapidly with time, as if dissolving in the cytoplasm, in a manner similar to 

 the wrinkles in the contracting tail region of the amoeba [4, 5, 7]. 



The disappearance of surface area of the vacuolar membrane at these 

 regions of increased thickness of the cytoplasm would be accompanied by 

 the simultaneous release there of anything adsorbed on the membrane. 



The small vacuoles which usually accompanied the wrinkles and which 

 suddenly formed as the depth of cytoplasm increased could be observed 

 growing in size, then moving to coalesce with the large vacuole of the cell. 

 In the presence of neutral red, concentrated dye could be observed in these 

 small vacuoles. 



