Chapter VIII — 125 — Active Relations 



He suggests a mechanism operative by a sudden increase in permeability, 

 whereby water, and solutes, are permitted to leak out. This would not ex- 

 plain plasmolysis in distilled water. An active contraction of the protoplast 

 seems to be a justified assumption in many instances. 



The phenomenon to which the name "vacuolar contraction" has been 

 given (KiJSTER, 1926), although differing fundamentally from plasmolysis, 

 deserves discussion at this point. This is a marked swelling of the proto- 

 plasm with reduction in vacuolar size but with no essential change in cell 

 volume nor separation of cytoplasm from the wall. It is not a simple 

 osmotic process, since it can occur in a hypotonic environment as a result of 

 some sort of stimulation. While onion scale epidermis and Elodea leaves 

 have been preferred materials, the reaction may be observed in many types 

 of cells (KiJSTER, 1940). Arens and de Lauro (1946) report that "hy- 

 dropot" cells in the epidermis of Sagittaria leaves offer favorable material. 

 Contraction may be experimentally induced by immersing sections or whole 

 leaves in 1 : 1000 neutral red solutions {pH 7 to 10) for a short time. The 

 vacuoles shrink often to a small fraction of their original size. 



Apparently the increase in protoplasmic volume occurs at the expense of 

 the vacuolar contents. Akerman (1917) was able to demonstrate an 

 osmotic pressure increase of 5 atmospheres in vacuoles of Drosera rotundi- 

 folia tentacles, due to vacuolar contraction. Arens and de Lauro {loc. cit.) 

 support the theory that water alone is absorbed by the cytoplasm, the tono- 

 plast restraining the passage of solutes. This is based only on the retention, 

 during the process, of neutral red or of anthocyanin in the vacuole. Hart- 

 MAiR (1937) found that in Elodea such a transfer of water from vacuole 

 to cytoplasm occurred, but that in onion epidermis there was a decrease in 

 plasmolytically determined osmotic pressure, hence the cause of contraction 

 could have been a sudden increase in permeability of the tonoplast. 



The underlying mechanism could well be an increase in the imbibition 

 pressure of the protoplasm. Arens and de Lauro prefer to regard it as 

 the result of oxidation-reduction processes, since many of the conditions 

 leading to vacuolar contraction are associated with respiration — /^H, wound- 

 ing, age and activity of the cells, and the specific effect of neutral red. 



The use of "cap" plasmolysis (page 88) as a device to study the effect 

 of ions on protoplasmic hydration has been made by Hofler (1939) and 

 his associates Kaiserlehner (1939) and Houska (1940). Swelling of 

 the cytoplasm of onion epidermis to 5-10 times the original volume may be 

 experimentally induced by the use of hypertonic solutions of K, Na, or Li 

 salts. Ca, Sr, and Ba salts do not act in this way. Accompanying effects 

 are an increased fluidity of the protoplasm and a cessation of streaming. 

 It is of interest that the reaction is entirely reversible. According to Hofler 

 the K salt might raise cytoplasmic hydration by becoming chemically bound 

 to some protoplasmic building block, resulting in an enhanced attraction 

 between this component and water. The cap plasmolysis method should 

 prove valuable in other studies of this kind. 



There is a great deal of physiological interest in both stimulative plas- 

 molysis and vacuolar contraction, since they clearly demonstrate the con- 

 tractile nature of the protoplasm and its wide variation in capacity to im- 

 bibe water. In certain instances the action might be interpreted as a defense 

 mechanism where the cell is exposed to adverse conditions. The possibility 

 should not be excluded that such variation may be associated with normal 

 functioning of certain cells. The observations do suggest, however, that 



