SPECIAL REAGENTS 191 



sections or leaf sections. There are several good formulae for safranin 

 stains (Sec. 184). 



M. Schnitzels macerating mixture. This mixture is used to disintegrate 

 tissues (e.g. wood) to obtain individual cells. The material to be treated 

 should be in small bits cut lengthwise. Place the sections in a test 

 tube and pour on just enough strong nitric acid to cover the material. 

 Add a few crystals of potassium chlorate and heat gently until bubbles 

 are given off and the substance treated becomes white. If violent 

 action occurs and abundant reddish fumes are evolved, repeat the 

 operation with fresh bits of the substance to be macerated, using less 

 chlorate. The process must be conducted out of doors or under a hood, 

 as the acid vapors produced are very corrosive and injure microscopes 

 and most metallic apparatus. When the maceration is finished the 

 fibrous material left should be thoroughly rinsed with successive por- 

 tions of water until all traces of acid are removed. It may then be 

 teased apart with needles and preserved in glycerin, and portions 

 mounted for examination as required. 



N. Sugar. Cane sugar is used in the preparation of solutions for the 

 culture of pollen tubes (Experiment XLII), algse (Sec. 200), and germi- 

 nating spores of mosses and ferns (App. 19 and App. 20). 



Solutions of the required strengths can be made by v^ighing out 

 the necessary amounts of granulated sugar and adding to measured or 

 weighed amounts of tap water. One and a half per cent of gelatin 

 may, with advantage, be added to most of the solutions for the culture 

 of pollen tubes. 



KILLING AND FIXING 



171. The principles of killing and fixing. Fixing is the preservation of the 

 structure of protoplasm immediately after death as nearly as possible like 

 that of the living cell. Killing and fixing are generally accomplished by the 

 same fiuid. Fixing agents have in their composition elements (as chromium, 

 osmium, platinum, etc.) which living protoplasm normally never or but 

 rarely encounters, or severe combinations of poisons that are utterly foreign 

 to it. The ability of a killing fluid to fix undoubtedly rests on its power 

 to subject protoplasm to a shock so sudden and great that there is little or 

 no time for gi'eat structural changes to take place, while the reagent itself 

 must not cause disorganization. Fixed material must later be preserved, a 

 process involving quite different methods and reagents (Sees. 177, 178). 



172. Chrom-acetic acid. The combination of chromic and acetic acids in 

 various proportions has proved to be a very satisfactory general fixing agent, 

 and is among the cheapest. Three grades of chrom-acetic acid will be found 



