188 



SCIENCE 



[N. S. Vol. XXXIV. No. 867 



increase the osmotic pressure inside the cell 

 until the latter equalled the external pressure. 

 In consequence the protoplast would expand 

 and return to its original condition. Overton 

 found that salts in general produce plasmolysis 

 which is not followed by expansion of the 

 protoplast. He therefore concluded that salts 

 are unable to penetrate. 



A repetition of Overton's experiments on 

 Spirogyra, using the same criterion of pene- 

 tration which he employed, has led me to the 

 opposite conclusion. In my experiments with 

 salts of NH,, Cs, Eb, Na, K, Li, Mg, Ca, Sr 

 and Al, the protoplast which is plasmolyzed 

 and left in the solution expands again to its 

 normal size, showing that all these salts read- 

 ily penetrate the protoplasm. 



I cite for illustration an experiment with 

 JSTaCl for the reason that this is very gener- 

 ally employed as a plasmolyzing agent. Fila- 

 ments of Spirogyra were placed in a .4M NaCl 

 solution. Within two minutes the protoplasts 

 of most of the cells were so far plasmolyzed 

 that they no longer touched the end walls of 

 the cells. Several of these were accurately 

 sketched with the camera lucida and kept 

 under continuous observation. In the course 

 of ten minutes several of them had begun 

 to expand and in thirty minutes all had ex- 

 panded so as to completely fill their respect- 

 ive cells. To avoid the injurious action 

 of the salt, the filaments were then trans- 

 ferred to .18M CaCl, solution and this was 

 gradually diluted until its osmotic pressure 

 was not greater than that of tap water. The 

 cells were then transferred to tap water. They 

 were examined the next day and found to be 

 alive. On being placed in .4M NaCl they were 

 plasmolyzed and afterward expanded as before. 



Recovery from plasmolysis is about as rapid 

 in KCl as in NaCl, while in CaCl. it is much 

 slower. On the other hand, in CsCl it is 

 much more rapid than in ISTaCl. In a subse- 

 quent paper the behavior in the various salts 

 will be fully described. 



Certain facts may be worthy of mention 

 which tend to obscure these results and which 

 may have caused them to be overlooked. 



In the experiment just described the cells 



were transferred to a favorable solution as 

 soon as expansion was complete. If this pre- 

 caution be neglected and the cells be allowed 

 to remain in the NaCl solution the injurious 

 action of the salt soon causes the protoplast 

 to shrink. In salts which are more toxic than 

 NaCl this shrinkage may be more rapid and 

 more pronounced. This shrinkage, which I 

 have called false plasmolysis,^ may also be 

 produced by very weak (hypotonic) solutions 

 and has nothing to do with plasmolysis but 

 may simulate it in very misleading fashion. 

 If the cells are not continuously observed but 

 only examined at intervals the expansion of 

 the protoplast may be easily overlooked, and 

 the subsequent shrinkage may be easily mis- 

 taken for plasmolysis. 



A further necessary precaution is the ob- 

 servation of the same individual cell during 

 the course of the experiment. To provide for 

 this and at the same time to keep the concen- 

 tration of the solution unchanged, a variety 

 of devices was employed which will be de- 

 scribed elsewhere. 



Cells which expand promptly if only slightly 

 plasmolyzed may not expand at all if severely 

 plasmolyzed. 



Some kinds of Spirogyra are wholly un- 

 suited for these experiments because they are 

 quickly injured by the salts (or by distilled 

 water made in metal stills if this be used for 

 solutions) in such a way that they expand 

 poorly or not at all. 



Another proof of the penetration of a salt 

 is illustrated by the action of CaCl, and NaCl. 

 A portion of a Spirogyra filament was plasmo- 

 lyzed in .2M: CaCk but not in .195M CaCl,. 

 A .29M NaCl solution has approximately the 

 same osmotic pressure as a .2M CaCl^ solu- 

 tion. But on placing another portion of the 

 same Spirogyra filament in a .29M NaCl so- 

 lution the expected plasmolysis does not occur 

 and it is impossible to plasmolyze the cells 

 until they are placed in .4M NaCl. It would 

 appear that this diiference between the be- 

 havior in CaCk and NaCl is caused by the 

 more rapid penetration of the latter. This 

 supposition is in perfect accord with the con- 

 ^ Cf . Bot. Gazette, 46 : 53, 1908. 



