ing agent for most plant tissues. With carbolic and hydrochloric 

 acids, lignified cells become yellowish green. The test is best made 

 by adding a few drops of concentrated HC1 to some crystals of car- 

 bolic acid, warming slightly, and when cold, add HC1 enough to 

 dissolve any crystals that may have separated out. This gives a 

 solution of crystals in just enough HC1 to dissolve them, and in 

 this the tissue is placed. (See Zimmermanu's Microtechnique, p. 

 145.) 



Chromic Acid. 



Chromic acid in strong solutions dissolves the cell wall rapidly 

 except in those cases where it may be cutinized, silicified, or corky. 

 If the action is allowed to continue, the cutinized wall will finally 

 dissolve. In dilute solutions the acid causes the cell wall to swell 

 and often brings out very clearly the markings or stratifications, as 

 in those of starch grains. Chromic acid is sometimes used in dilute 

 solutions as a hardening agent. 



Cuprammonia. 



This is the so-called Schweizer's reagent and is effective only 

 in fresh solutions. It is prepared by adding to an aqueous solu- 

 tion of copper sulphate some sodium Hydrate, until a precipitate of 

 copper hydrate is formed. The precipitate is filtered out and 

 washed with hot water, after which it is dissolved in as little 

 ammonia as will take it up. It forms a deep blue solution and will 

 dissolve quickly cotton fibers. Cell walls of pure cellulose swell 

 and are readily dissolved by the solution, but, if they contain 

 lignin or suberin, the reagent will not act until these substances 

 are removed in some way, e. g., by Schulze's maceration method. 



Cleaning Mixture. 



A cleaning mixture that works rapidly and removes balsam at 

 once from the slide is made by adding two parts of strong HNO 3 

 to three parts of concentrated H 2 SO 4 . The mixture must be kept 

 covered as the fumes are very disagreeable. This mixture cleans 

 glass very quickly and does not injure it. A dichromate cleaning 

 mixture is made by dissolving 200 grams of potassic dichro- 

 mate in 1000 c. c. of water and then adding to the mixture 1000 



