32 METHODS IN PLANT HISTOLOGY 



d) Corrosive sublimate and picric acid (Jeffrey's solution). — 



Corrosive sublimate, saturated solution in 30 per 



cent alcohol 3 parts 



Picric acid, saturated solution in 30 per cent 



alcohol 1 part 



It would be worth while to try other combinations. 



IODINE 



Iodine is well known as an antiseptic. It is also a good fixing agent 

 for unicellular, colonial, and filamentous forms. It penetrates rapidly. 



To a saturated solution of potassium iodide in distilled water, add 

 iodine to saturation. Filter and dilute with distilled water until the 

 solution has a rich brown color. For fixing, dilute still further to a 

 light-brown color. The solution fixes in 10-24 hours, but material may 

 be left in it for several days. Wash thoroughly in tap water which has 

 stood long enough to give off all excess of air. If the staining of the 

 starch does not disappear, a | per cent solution of tannic acid in water 

 will remove any excess color. 



FORMALIN 



Formalin is an excellent preservative. It has been mentioned al- 

 ready as an ingredient in several formulas. Commercial formalin has 

 a strength of 40 per cent. Throughout this book, a 2, 4, or 6 per cent 

 formahn is understood to mean 2, 4, or 6 c.c. of commercial formalin 

 to 98, 96, or 94 c.c. of water, alcohol or any other ingredient. Com- 

 mercial formalin is sure to contain some formic acid. For most pur- 

 poses, it is neither necessary nor desirable to remove the acid. For 

 studying the origin of vacuoles, it is necessary to have neutral forma- 

 lin, which can be secured from commercial formalin by distillation. 

 Place some sodium bicarbonate in a flask of formalin and distil by 

 heating over a Bunsen flame. It is not worth while to distil more than 

 is needed for immediate use, since the formic acid soon reappears. 



For filamentous algae and fungi a 3-6 per cent solution of the or- 

 dinary commercial formalin in water is very good. Material is left in 

 the solution until needed for use. For marine algae, sea water should 

 be used instead of fresh water. Both marine and fresh-water material 

 should be washed for an hour in fresh water before staining. Material 

 of Polysiphonia, left in a 12 per cent solution of formahn for 10 years, 



