634 R- J- GOLDACRE 



" shimmering " or vibratory activity of the red cell membrane was reported 

 by Pulvertaft [12], using a special optical technique, which may represent 

 minute contractions of various small regions of the cell membrane. This 

 "shimmering" could be abolished by o- 1 m fluoride, which also abolished 

 the uptake of potassium ions, indicating a possible connection between 

 the two [i]. Recently, also, ATP has been implicated in the maintenance 

 of the biconcave shape of the red cell [10]. 



In plant cells, a contraction occurring in the membrane at one end of 

 the vacuole in root hairs appeared to be associated with the release of 

 neutral red into the vacuole at that place [5], when the cells were immersed 

 in the dye solution. That the vacuole is implicated in the accumulatory 

 mechanism is indicated by the observations of Brown [2] who showed that 

 active uptake of potassium occurred in the roots of higher plants only at 

 and above the zone of elongation, where vacuoles were present. 



In this communication I should like to describe an expansion-contrac- 

 tion cycle in the vacuolar membrane of giant NiteUa cells, which appears to 

 be associated with an accumulatory mechanism. 



In order for an accumulatory mechanism to be efi^ective, there should 

 ideally be two processes: {a) concentration of material, and {b) a valve 

 action or one-way movement, so that accumulated material does not leak 

 away. However, (6) is not absolutely necessary, if the rate of accumulation 

 is high, and the rate of leakage lower ; some accumulation would still occur 

 and this appears to be the situation in the accumulation by amoebae of 

 neutral red, which can in time be washed out of the cells by water. Nitella 

 cells appear to provide both these mechanisms. 



2. Internodal cells 



The large chloroplast-lined internodal cells of NiteUa are not particu- 

 larly active in accumulating neutral red, accumulating about as much in a 

 few hours as the more active rhizoids accumulate in a few minutes from 

 the same solution. A casual inspection of the cyclosis reveals no obvious 

 regions of cytoplasmic contraction, the cytoplasm circulating at a uniform 

 speed of about 100 microns per second in a slow spiral path up one side 

 of the cell and down the other. In some of the cells, however, interesting 

 phenomena occur at the ends where the cells are attached to their neigh- 

 bours. Usually, groups of small cells cover and obscure the junction and 

 it is necessary to search through many specimens until a cell favourably 

 placed for observation can be found (Fig. i, B). 



In some of these, it can be observed that the layer of cytoplasm at the 

 end is thicker (^3, Fig. i) than that running into it {d^), and this thicker 

 layer moves more slowly. This means that the surface area of the vacuolar 

 membrane is contracting at this point. For example, consider unit volume 



