General Remarks on Staining 69 



carefully used, gives a rich purple color, but a careful examination 

 will often show that in the same preparation some structures stain 

 purple, while others stain red. Does this mean that the purple and 

 red structures must have a different chemical composition ? Many 

 people believe that structures which stain differently with a given 

 stain must be chemically different, but they readily agree that struc- 

 tures which stain alike are not necessarily similar in chemical com- 

 position. Chromosomes of dividing nuclei and lignified cell, walls 

 stain alike with safranin; chromosomes and cellulose cell walls stain 

 much alike with Delafield's haematoxylin; but everyone recognizes 

 that the chromosome is very different in its chemical composition 

 from either the cellulose or the lignified wall. 



However, in an indirect and somewhat uncertain way, one can 

 infer the nature of certain structures from the staining. For instance, 

 if sections of various objects have been stained with safranin, we may 

 draw the following inferences with more or less confidence: if cells 

 in the xylem region of a vascular bundle stain red, their walls are 

 lignified; if cortical cells, which may appear quite similar in trans- 

 verse section, stain red, they are likely to be suberized; if the outer 

 walls of epidermal cells stain red, they are cutinized; but if the outer 

 boundary of the embryo-sac of a gymnosperm stains red, it is chitin- 

 ized. Of course, these inferences can be made only because the 

 various structures have been tested by more accurate methods. 



Whatever doubt or uncertainty there may be in regard to theories 

 of staining or in regard to the value of stains as a means of analysis, 

 there is no doubt that stains are of the highest importance in differ- 

 entiating structures, and in bringing out details which would other- 

 wise be invisible. 



PRACTICAL HINTS ON STAINING 



The number of stains in the catalogs is becoming so great that 

 it is impossible to become proficient in the use of all of them. As 

 we have already intimated, it is better to master a few of the most 

 valuable stains than to do indifferent work with many. An experi- 

 enced technician knows that it is impossible to judge from a few 

 trials whether a given stain or combination is really valuable or 



