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W. J. V. OSTERHOUT 



soluble in lipoid would penetrate the living cell. Overton's 

 experiments seemed to show that this is really the case, but more 

 recent experiments have brought so many exceptions to light 

 that it no longer seems to hold. 



The penetration of substances other than dyes has been shown 

 in various ways. Some cells contain natural indicators by which 

 the entrance of acids or alkalies is e\ddenced ; others can be stained 

 by neutral red or some other indicator before applying the acid 

 or alkali. Certain substances cause precipitates within the cell 

 or produce other visible changes. FaiUng this the penetration 

 of the substance may sometimes be shown by the subsequent 

 application of a suitable reagent. 



To most of these experiments the objection could be made 

 that the reagents altered the permeability of the protoplasm and 

 that it was not shown by subsequent and sufficiently prolonged 

 observation that the cells were wholly uninjured by the exper- 

 imental treatment. 



In order to overcome these objections experiments were per- 

 formed on actively growing root hairs. As individual hairs were 

 kept under observation for several days before and after being 

 tested for permeability, it was possible to say with certainty 

 whether injury had ocurred. By growing the roots in distilled 

 water root hairs were obtained which were free from crystals. 

 On placing them in tap water or in dilute solutions of calcium 

 salts, crystals of calcium oxalate soon made their appearance 

 inside the cells, thus proving the penetration of the calcium salts, 

 These cells subsequently grew and developed in a perfectly 

 normal manner. 



The absorption of oxygen and the excretion of carbon dioxide 

 and other substances under normal circumstances shows that the 

 plasma membrane is permeable to these substances. But the 

 absorption of food substances is usually too slow to serve as a 

 valuable criterion. 



to this precaution. Such tests are usually made by shaking the substance in a 

 medium containing drops of oil or finely divided lecithin or cholesterin, but this 

 method is unsatisfactory, since no distinction is made between the part played by 

 adsorption and that played by solution. 



