26o PRACTICAL BOTANY 



Another method (Gage) is to dissolve a quantity of picric acid 

 in 100 parts of water, and an equal quantity of carmine in 50 

 parts of solution of ammonia ; these are then mixed, filtered, 

 evaporated to dryness, and the residue dissolved in 100 parts 

 of water. 



Picro-carmine is used especially for staining nuclei, the 

 staining being more uniform than when carmine alone is used ; 

 it has this further advantage, that a prolonged exposure to it 

 does not produce overstaining, as is the case with the other 

 preparations of carmine. The objects should be previously 

 kept for some time in absolute alcohol. If it be desired to 

 retain the double staining which this reagent produces, the 

 sections must be mounted at once in glycerine ; but if the 

 carmine staining only is required, the sections must be washed 

 in water, which will dissolve out the picric acid. When stained 

 sections are mounted in glycerine, a small quantity of picro- 

 carmine must be added to the glycerine in order to preserve 

 the colours. 



The various preparations of carmine can be used as well for 

 tissues which have been hardened in chromic, picric, or osmic 

 acid, as for fresh tissues, but the former stain less readily. 



Picro-nigrosin : make a saturated solution of picric acid, add 

 crystals of nigrosin, and allow them to dissolve : steep the speci- 

 men in it, and allow time for slow staining ; this reagent may 

 be used for simultaneous fixing and staining of delicate tissues, 

 and is especially recommended in the preparation of Spirogyra 

 and other Algae, and for Fungi. 



{ Potash may either be used in a dilute solution (1-5 per cent.) 

 or in a strong solution of water. A dilute solution is commonly 

 used as a clearing agent : it causes cell-walls and starch-grains 

 to swell, especially when heated, and it dissolves sphere-crystals 

 of inulin, crystalloids, and most aleurone-grains, and saponifies 

 fats. It gives a reddish colour to cells in which tannin is present. 

 A strong solution may be used as a test for suberin : when 

 sections of cork are boiled in strong potash, the suberin escapes 

 in the form of yellow viscid drops ; when the sections are only 

 slightly warmed in the solution, the cuticularized walls assume a 

 yellow colour. 



