GENERAL METHODS. 19 



that of glycerine and that of dammar lac, so that cell-mem- 

 branes and starch grains stand out pretty sharply in it. 



A disadvantage of this medium lies in the fact that it be- 

 comes solid very slowly. In order to secure a firm attach- 

 ment of the cover-glass to the slide, which is often very de- 

 sirable in studies with immersion lenses, one may apply a 

 heated metal wire to the edge of the cover-glass, as Vosseler 

 (II, 297) has done. The same object may also be attained, 

 according to Pfeiffer (I), by encircling the cover with Canada 

 balsam. 



Various stained tissues, especially carmine, haematoxylin, 

 and saffranin preparations, may be excellently preserved in 

 Venetian turpentine, according to Vosseler. In my own 

 experience, however, acid fuchsin seems to be more poorly 

 preserved in it than in Canada balsam. 



VI. Staining" of Living Tissues. 



28. As has been shown especially by the researches of 

 Pfeffer (II), it is possible in very many cases to cause living 

 plants and parts of plants to take up certain coloring mat- 

 ters. This so-called live staining is not only of great impor- 

 tance for the study of the transportation of material within 

 the vegetable organism, but has also led to some interesting 

 results concerning the morphology of the cell, and should 

 certainly be capable of still wider application. 



For the success of live staining it is of primary importance 

 that the staining solution used should exercise no injurious 

 effect upon the objects concerned. Since the aniline colors 

 generally act as poisons on plant-cells, it is necessary to use 

 them in very dilute condition, when they affect the cell very 

 little or, in general, not injuriously. An evident staining 

 can, it is clear, only take place when the stain is stored up 

 by certain constituents of the cell. This is generally the 

 case when the osmotic balance between the cell fluid and 

 the surrounding staining solution is constantly destroyed by 

 a chemical metamorphosis of the stain taken up. But to 

 make possible in this way the storage of large quantities of 

 staining material, it is also necessary to furnish the objects 



