SPECIAL METHODS. 239 



colored cell-sap are also very useful, as, for example, the 

 epidermal cells of the red lower surface of the leaf of Tra- 

 descantia discolor. The earliest beginnings of plasmolysis 

 are easily observed in such cells. 



431. In many cases the plasmolysis may be made plainer 

 by an artificial coloring of the cell-sap. In cells containing 

 tannic acid this may be accomplished by exposing the ob- 

 jects on the slide, in a drop of the plasmolyzing solution, to 

 the vapor of osmic acid for a time, according to the method 

 described in 308. The protoplasts are then completely 

 fixed in their original position and are much easier to ob- 

 serve on account of the browning or blackening of the cell- 

 sap. Plasmolysis may also often be made plainer by pre- 

 liminary staining intra vitam. 



Finally, by adding an indifferent pigment, like eosin, to 

 the plasmolyzing fluid, a difference in color between it and 

 the cell-sap may be produced, 



To test for the presence of a living plasma-body within 

 the vessels, Th. Lange (I, 404) injected large pieces of tissue 

 with a 5$ solution of saltpeter under the air-pump, and 

 then added a dilute solution of picric acid to fix the tissues. 

 After washing in water and dehydrating in alcohol, he 

 placed them in clove-oil. Very good thin sections were 

 prepared from the material so treated, and in these the pro- 

 toplasm was stained with borax-carmine or eosin. 



Abnormal Plasmolysis. 



432. As was shown by H. de Vries (I), many solutions 

 cause the complete killing of the protoplasm and its inclu- 

 sions, with the exception of the inner plasma-membrane 

 bordering on the cell-sap, which is not changed in its 

 osmotic relations, so that it still completely excludes the 

 cell-sap (cf. Fig. 60). If this proceeding, which de Vries 

 calls abnormal plasmolysis, does not justify such far-reaching 

 conclusions as he has drawn from it (cf. on this point Pfeffer 

 VIII, 240), yet abnormal plasmolysis may do good service 

 in many investigations. 



