Chapter XIII -145- Vital Staining 



minimizes this difference which formerly seemed inexplicable. In- 

 deed, this work has shown that Saprolegnia which grows with the 

 dye accumulated in its vacuoles can itself, in the course of develop- 

 ment, excrete the dye from its vacuoles into the external medium 

 under some conditions. This fungus, therefore, has a behavior 

 intermediate between that of the phanerogams and that of the 

 other fungi. It is not certain that the phanerogams themselves 

 may not be capable of excreting the dye in their vacuoles to the 

 exterior, but this is done with difficulty and under special 

 conditions. 



The purpose for exposing here the essential data of these still 

 little known investigations before beginning the study of the vacu- 

 oles is to emphasize the following points. Among the dyes which 

 stain the vacuoles, neutral red, Nile blue and cresyl blue are only 

 very slightly toxic and, in general, bring about only a staining of 

 the vacuoles. In addition, the staining of the vacuoles can be pro- 

 duced only during the life of the cell and is essentially a vital phe- 

 nomenon. Furthermore, it is proved experimentally that neutral 

 red is much the least toxic of these dyes and that it is never retained 

 by the cytoplasm and, except in rare cases, accumulates only in 

 the vacuoles. It is a stain which is almost specific for vacuoles 

 and is the most useful dye for the study of these elements. It has, 

 besides, the advantage of not being reduced by the cells, as are 

 cresyl blue and Nile blue (in the case of the fungi). Neutral red, 

 therefore, is indisputably the best vital stain for vacuoles. 



This being the case, the importance of the method of vital stain- 

 ing in the study of the vacuoles is readily appreciated. This vital 

 staining makes it possible not only to observe the vacuoles in 

 preparations between slide and cover glass but also to follow, at 

 least in most plants, the entire life history of these elements in 

 cultures which are growing on media to which vital dyes have been 

 added. This method of staining living material has led to im- 

 portant discoveries which we shall now set forth. 



