DOUBLE -STAINING. Ill 



staining of cross-sections of Zeci Mais with the object of bringing 

 out its structure, each stain picking out some special structural 

 feature. According to the stains used, the process can be by suc- 

 cessive treatment with different stains, or simultaneous staining by 

 means of a suitable mixture. We can obtain a very beautiful 

 double staining if we allow the sections to lie for a little while in 

 watery iodine-green solution and then for a somewhat longer time 

 in Paul Mayer's alum -carmine. Instantaneous double staining can 

 be obtained by means of picro-nigrosine or picro-aniline blue. In 

 the carmine iodine -green preparation the unlignified membranes 

 are stained with the carmine, the lignified walls by the iodine- 

 green. In picric double staining the picrin dyes the lignified, 

 the nigrosine and aniline blue the unlignified membranes. The 

 fixed cell-contents have taken colour from the carmine, nigrosine 

 or aniline blue respectively. 



In some cases stained preparations which are to be pre- 

 served in glycerine- jelly should have the stain fixed, so that the 

 glycerine may not dissolve it out. For amydo colours, such as 

 methyl-blue or fuchsin, a mordant of tannin and afterwards tartar 

 emetic should be used ; for oxyazo colours, such as ponceau, a 

 mordant of zinc chloride. In many cases a 10 per cent, solution 

 of alum does well as a mordant. Many amydo-azo colours, such as 

 benzo-purpurin, fix in an alkaline bath without needing any 

 other mordant. What mordant to use in any special case, pro- 

 ducing no " muddiness," but clear differentiation, should always 

 be first determined. 



Permanent double staining can be obtained by the use of 

 methyl-violet and carmine. The sections are first treated for 

 a few minutes with a fully concentrated alcoholic solution of 



braues both lignified and unlignified, but with different degrees of intensity 

 or in different tones or shades of colour. If permanent preparations are 

 desired, important limitations arise from the relative durability of colours in 

 the various mounting media. The list of permanent stains is small. Some- 

 times a single stain will do all that is necessary, in which case the student 

 may restrict himself to the use of safranin, which differentiates tissues very 

 well, the lignified membranes more cherry-red, the unlignified membranes 

 more brown-red ; and the staining is permanent. Fuchsin (magenta) also is 

 very valuable as a fairly permanent stain for lignified membranes. If two 

 stains are used consecutively, it is generally desirable to use first that which 

 stains the lignified membranes ; wash the preparation then in water (or in 

 some cases alcohol) as long as is necessary, and then treat with the second 

 stain. [ED.] 



