REAGENTS. 103 



ALCANNIN. Macerate 20 grams of alcanet root in 100 cc. of 

 90 per cent, alcohol for a week and filter. Dilute the tincture 

 with an equal volume of water just before using and immerse 

 the sections for several hours. The reagent is a test for fats, 

 resins and volatile oils, which assume a red color. Cutinized 

 and suberized cells are also stained red, though not so deeply. 



AMMONIUM FERRIC ALUM. A saturated aqueous solution is 

 used as a test for tannins, which form a bluish-black or green- 

 ish-black precipitate. It should be remembered, however, that 

 occasionally other substances, usually related to the tannins, 

 may be present, which are capable of forming dark-colored pre- 

 cipitates with ferric salts. 



FIXING AND HARDENING REAGENTS. 



ALCOHOL. This is universally used for hardening plant tis- 

 sues. It hardens by abstracting water. Strong alcohol pos- 

 sesses also in a high degree the power of fixing the protoplasmic 

 contents of cells. Some plant organs may be placed at once 

 into absolute alcohol, while other more tender parts must be 

 placed at first into weak alcohol (60 per cent.) , then into grades 

 of increasing strength, as 70 per cent., 90 per cent., absolute 

 alcohol. Tissues may be kept in alcohol for any length of time. 

 They acquire the best condition for cutting if they are placed 

 in a mixture of equal volumes of water, absolute alcohol and 

 glycerin 24 hours before sectioning. Other less frequently used 

 solutions are 



CHROMACETIC ACID. A mixture of 1 part of 0.1 per cent, 

 acetic acid and 1 part of 0.2 per cent, chromic acid solution. It 

 is a fixing reagent and objects must remain in it from several to 

 24 hours. They are then thoroughly washed in water and hard- 

 ened in alcohol. 



CHROMIC ACID. A 1 per cent, solution is used for the same 

 purpose as the previous solution. All chromic acid mixtures 

 should be kept in the dark, as sunlight decomposes them. 



OSMIC ACID. A 1 per cent, solution fixes protoplasm imme- 

 diately. Objects may remain in it from a few seconds to sev- 

 eral hours according to their nature, and are then washed thor- 

 oughly in water and hardened in alcohol. 



PICRIC ACID. In concentrated aqueous or 50 per cent, alco- 

 holic solution for algae and higher plants. 



SOFTENING REAGENTS. 



In some exceptional cases objects are too hard for sectioning 

 and, therefore, must be rendered soft before they can be studied. 

 Such objects are wood, hard seeds, barks, dried drugs. In some 



