62 METHODS IN PLANT HISTOLOGY 



finished mounts upon a white background in the direct sunHght. 

 When the desired differentiation has been reached, it is better to avoid 

 direct sunHght, although the mounts do not seem to fade in the or- 

 dinary hght of a room. 



Except for special purposes, it is better to use this stain in combina- 

 tion with blue, green, or violet. 



Gentian violet. — Dissolve in the anilin oil solution as directed in 

 the general formula. Although it does not keep as well as the aqueous 

 solution, it stains better, especially when dealing with the achromatic 

 structure of the mitotic figure. Often the brilliancy of the stain can be 

 increased by leaving the slide for about 5 minutes in a 1 per cent aque- 

 ous solution of permanganate of potassium before applying the stain, 

 or in Gram's iodine solution (1 g. iodine, 2 g. potassium iodide, 300 c.c. 

 water) after staining in violet. 



The greatest objection to the aqueous and anilin-oil solutions of 

 gentian violet is that the stain washes out so rapidly in alcohols that 

 it is impossible to run the slide up through the series. The usual prac- 

 tice is to dip the slide in water to remove most of the stain and thus 

 avoid carrying it into the alcohol : then transfer directly from water to 

 95 per cent alcohol, allowing the alcohol to act for only 2 or 3 seconds, 

 then allow the absolute alcohol to act for 5 or 6 seconds, and then, 

 while the stain is still coming out in streams, begin the treatment with 

 clove oil. Holding the slide in one hand, pour on a few drops of clove 

 oil, and immediately drain off in such a way as to carry off the alcohol. 

 This clove oil should not be used again. Then flood the slide repeat- 

 edly with clove oil, pouring the clove oil back into the bottle. A 50-c.c. 

 bottle of clove oil is large enough. About 100 mounts can be cleared 

 with 50 c.c. of this oil. The clove oil is a solvent of gentian violet, but 

 it dissolves the stain from some structures more rapidly than from 

 others; e.g., the stain may be completely removed from the chromo- 

 somes while it is still bright in the achromatic structures. As soon as 

 the stain is just right, drain off the clove oil and put the slide into a 

 Stender dish of xylol for a couple of minutes. The xylol will soon take 

 on an amber color, but this will not reduce its efiiciency in clearing; on 

 the contrary, its efficiency will improve. However, the least trace of 

 clove oil, carried over into the balsam, will finally cause the stain to 

 fade. Therefore, transfer the slide to fresh, clear xylol and let it re- 

 main for 2 or 3 minutes before mounting in balsam. 



As may be inferred from what has preceded, alcohol would soon ex- 



