APPENDIX A 515 



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 I 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 in water. A dilute solution is commonly 

 used as a clearing agent (p. 49) : 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 (p. 39). 



A concentrated solution of caustic potash in alcohol is some- 

 times used with good effect in the preparation of apical meristems, 

 but specimens so treated cannot be permanently kept. 



Potassium Acetate (see Acetate). 



Potassium Bichromate is used in dilute solution in water as 

 a test for tannin, which it colours dark brown : the 1 per cent, 

 solution in water may also be used for hardening tissues. 



Potassium Chlorate is used together with nitric acid as a 

 macerating agent, and as a test for suberin (see below, Schulze's 

 Macerating Fluid). 



Eussow's Callus-Reagent (see above, Callus- reagent). 



Safranin. This may be used in solution in absolute alcohol. 

 It is especially adapted for staining sections which have been 

 previously hardened with chromic or picric acid ; it is not so 

 good for those which have been treated with osmic acid. The 



L L 2 



