Chapter VII 



89 



Osmotic Quantities of Cells 



Fig. 25. — Vacuole con- 

 traction in a cell of the fruit 

 flesh of Ligustrum vulqaire. 

 Redrawn from Strugger 

 (1935). 



tions should be buffered to a favorable pH value and physiologically bal- 

 anced with respect to uni- and bivalent cations. Toxic solutes should be 

 avoided as far as possible and temperature and 

 light should be regulated to avoid injury of the 

 cells. 



While plasmolytic methods are of interest in 

 the study of cell processes, plasmolysis is not of 

 normal occurrence in plant tissues. There may be 

 exceptions, e.g., spore formation in certain algae 

 (Gross, 1940). Generally, it is considered to be 

 injurious to cells, either by rupture of the plasmo- 

 desmata connecting them and by rupture of what- 

 ever union exists between the unpitted wall and 

 the protoplasm, or by effecting certain colloidal 

 changes in the protoplasm. Scarth (1941) pre- 

 sents evidence that the degree to which a cell may 

 be plasmolyzed without undue injury "is deter- 

 mined by the point at which an irreversible stiffen- 

 ing, presumably coagulation, of the protoplasm 

 occurs," and that "the immediate cause of death 

 is usually the rupture of the rigid ectoplasm on 

 deplasmolysis." Bennet- Clark and Bexon 

 (1943) have shown that there is a rapid increase 

 in the respiratory rate on plasmolysis of beetroot 



cells. The question as to whether osmotic pressure 

 determined at limiting plasmolysis could be converted 

 to tlie value at normal volume merely by considering 

 volume change has always been troublesome to the 

 investigator. An increased respiration rate is one 

 of the first definite indications that marked changes 

 do occur during plasmolysis. Such a response may 

 effect, or may be the result of, other reactions which 

 conceivably are involved in the water balance of the 

 cell. 



Returning to the plasmolytic measurement of os- 

 motic pressure, it can be safely stated that the anom- 

 alies discussed above are not often observed when 

 proper precautions are taken and when suitable ma- 

 terial is employed. As a rule, best results are ob- 

 tained where the plasmolysis form is convex. 



The Plasmometric Method: — Introduced by 

 HoFLER (1917) this procedure is adaptable to cer- 

 tain types of cells. The technique involves rather 

 strong plasmolysis, followed by measurement of the 

 respective volumes of cell and protoplast. The value 

 of Og is obtained from the following relationship : 



Vol. of protoplast 



Fig. 26. — Vacuole 

 contraction in an onion 

 epidermal cell having the 

 vacuole strongly stained 

 with neutral red. Drawn 

 from a photograph by 

 Strugger (1935). 



Og = OP of plasmolyzing sol. X 



Vol. of turgorless cell 



The method is claimed to be highly accurate, and 

 may be more suitable for single cells than the limiting plasmolysis method. 

 However, there are certain disadvantages. Only cells of regular shape 



