Grafts et al. — 82 — Water in Plants 



An external solution which reduces the turgor of the cell to zero, but 

 does not cause plasmolysis, is said to be isotonic with respect to the cell sap ; 

 the cell assumes the condition of limiting plasmolysis. The osmotic pres- 

 sure of the bathing solution at this state is designated by the symbol Og. 

 In practice the condition of limiting plasmolysis is detected with difficulty ; 

 a slightly more advanced stage known as incipient plasmolysis wherein the 

 protoplasm first begins to draw away from the walls is used as a standard 

 reference state. Since this is taken for the condition of isotonicity, there is 

 by definition a slight difference between the true osmotic pressure of the 

 flaccid cell, and that measured at incipient plasmolysis. 



Limitations of the Plasmolytic Method: — Although the plasmolytic 

 method has been criticised as involving too many errors, many of the im- 

 portant cell-water concepts have been developed by its use. It has several 

 advantages: 1) measurements on single cells are possible, 2) intact living 

 cells may be investigated, 3) permeability and other types of studies may be 

 made in conjunction with the method. The following criticisms have been 

 ofifered to its use : 1 ) the actual measurement is restricted to the state of 

 incipient plasmolysis. Only by correcting for the change in volume may 

 values be computed for cells in any state of turgor. — 2) Permeability 

 changes may be involved, which, while of interest from the standpoint of 

 permeability, may bring about errors due either to absorption of the plas- 

 molyzing solute or to loss of cell solutes or both. — 3) It may be inapplic- 

 able to cells possessing large protoplasm : vacuole ratios (Oppenheimer, 

 1932a). — 4) Adhesion of the cytoplasm to the cell wall, at least for cer- 

 tain types of cells, allegedly may result in excessive osmotic pressure values 

 (BuHMANN, 1935). — 5) The cell wall may be so impermeable to the 

 solute of the plasmolyzing solution that it crinkles (Huber and Hofler, 

 1930; Pringsheim, 1931). — 6) The exact determination of incipient 

 plasmolysis is often difficult because of inability to observe the very first 

 separation of the cytoplasm from the wall. — 7) Mechanical shock from 

 cutting or isolation of sections may produce abnormal conditions in the 

 protoplasm (Lucke and McCutcheon, 1932). — 8) Sap released from 

 cut cells may exert injurious efifects on intact cells. — 9) There is no critical 

 point in the plasmolysis time curve (Ernest, 1935). — 10) Plastic stretch- 

 ing or shrinking of cell walls may invalidate the results. (Oppenheimer, 

 1930&). 



Because of the long list of possible errors, one may question the accuracy 

 of measurements made by this method. However, some of the criticisms 

 are unjustified ; errors involved in others are so small as to be insignificant. 

 Material may be selected that does not sufTer from all of the disadvantages 

 noted. Tissues with pigmented vacuoles have been popular because in- 

 cipient plasmolysis is easily recognized. The leaf epidermis of Rhoco dis- 

 color is a classical material. 



As a plasmolyzing substance, sucrose has been almost universally em- 

 ployed. It is nontoxic, does not appreciably penetrate the protoplasm, and 

 its physical and chemical properties are well known. Beck (1927), after 

 critically comparing sucrose with potassium nitrate solutions, concluded 

 that results obtained with the latter were unreliable because of an excessive 

 penetration into the cells. Many of the early plasmolytic data were ob- 

 tained through the use of KNOo. Peters (1942) states that "sucrose 

 appears to be more completely excluded from cells than any of the other 

 substances." To mannitol has been ascribed similar properties (Collander 



