ACTIVE TRANSPORT AND MEMBRANE EXPANSION-CONTRACTION CYCLES 637 



These small vacuoles are probably induced to form in the cytoplasm 

 by the sudden release into it of a high concentration of accumulated 

 material : it was observed, during work on amoebae, that neutral red 

 injected into the cytoplasm with a micropipette soon induced vacuoles 

 containing the neutral red to form in the cytoplasm ; similarly, the dye 

 accumulating naturally in the amoeba, when present in the external 

 medium, at first formed a diffuse cloud in the tail and shortly after became 

 segregated into vacuoles [4, 5]. Vacuoles appear to form, by some unknown 

 mechanism, as a result of high concentrations of various substances. 



This coalescence of the induced vacuoles with the large vacuole 

 occupying most of the cell would provide a valve action or one-way effect 

 for the accumulated dye, for the reverse phenomenon — the pinching off of 

 small pieces of the central vacuole — was not observed ; so that dye released 

 locally in high concentration by a contraction of the vacuole membrane, 

 on which it was adsorbed, would be captured in these small induced 

 vacuoles, and, by their coalescence with the large vacuole, be prevented 

 from returning to the external medium. 



It might be wondered whether the amount of material capable of being 

 adsorbed on the vacuolar membrane could make a significant contribution 

 to the cell's content when desorbed in a contraction. Suppose a monolayer 

 of adsorbed substance weighed one-tenth milligram per square metre (this 

 is about one-tenth of the weight of a protein monolayer). Then in a stream 

 of cytoplasm travelling at 100 microns per second and 30 microns wide, 

 3000 square microns will disappear per second, releasing 3 x io~^'' mg. to 

 the vacuole per second, and increasing the sap concentration by 0-3 p. p.m. 

 per second, or 2°o per day. Hence, if any substances, adsorbable on the 

 vacuolar membrane, succeeded in diffusing passively into the cell in low 

 concentration, they would be concentrated by the local contractions and 

 ultimately captured by the central vacuole by the coalescence with it of 

 the small vacuoles they induced. 



5. Summary 



In Nitella cells exhibiting cyclosis, small localized regions exist where 

 a continuous contraction of area of the vacuolar membrane occurs. These 

 are regions where a sudden fall in speed of streaming occurs, owing to an 

 increase in depth of the stream produced by some discontinuity in the 

 cells, resulting in accumulation of membrane, with wrinkling and its 

 ultimate dissolving into the cytoplasm. Any adsorbed substances would be 

 shed there. A valve action appears to be provided by the formation of small 

 vacuoles in the cytoplasm, apparently induced by the high concentration 

 of substances shed, and containing the substances in high concentration ; 

 these small vacuoles coalesce with the large vacuole in the cell. 



