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W. J. V. OSTERHOUT 



fectly the problem of preserving the normal permeability of the 

 plasma membrane. 



b. It is highly important to keep the same individual cell under 

 observation throughout the experiment and to make observa- 

 tions as frequently as possible. 



c. It is necessary to distinguish between true and false plas- 

 molysis/ and where this is impossible to reject the results in all 

 cases. 



d. Experiments in which the criterion of permeability is the 

 recovery of the protoplast {i.e., its expansion when left in the 

 solution) should be carried out at constant temperature, as recov- 

 ery is more rapid at higher temperatures. "VMien the temperature 

 cannot be kept constant each experiment should be accompanied 

 by numerous controls in which a part of the material is plasmolyzed 

 in sea water at each of the different temperatures employed. It 

 is, of course, important that plasmolysis should not be too great 

 and that all the cells should be plasmolyzed to the same degree. 

 Cells which recover promptly when slightly plasmolyzed may 

 recover very slowly or not at all when severely plasmolyzed. 



Ha\Tng found a reliable standard and satisfactory methods of 

 using it, the next step was to obtain uniform material. The 

 writer has found it very desirable to employ long filaments of 

 Spirogyra or Chaetomorpha which may be cut with a sharp scissors 

 into short pieces. A piece of each filament is tested in the 

 standard solution. In this way very uniform material may be 

 secured. The same procedure is used with Elodea and similar 

 material. 



* True plasmolysis is produced only by hypertonic solutions (those having a 

 greater osmotic pressure than the cell) but appearances closely resembling it may 

 be produced by dilute (hj'potonic) solutions of NaCl and many other substances, 

 or even by distilled water made in a metal still. These appearances may also 

 be produced in many marine plants by transferring the cells from seawater to 

 pure water taken directly from springs or ponds, or to pure distilled water (made 

 in a glass or quartz still with all precautions, and not toxic to sensitive root hairs 

 and other test objects). To such appearances we may apply the term "false 

 plasmolj'sis." False plasmolysis is usually distinguishable from true plasmolysis 

 by abnormal' appearances or by the length of time required to produce it, or by 

 the ability of the cell to recover its normal appearance when returned to its usual 

 environment. 



