APPENDIX A 511 



stains the protoplasmic cell-contents and not the cell-wall : it 

 stains also the callus which closes the perforations of the sieve- 

 plates during the winter in perennial plants. It is also used, 

 together with sulphuric acid, for demonstrating the continuity of 

 protoplasm through cell-walls : in order to do this a small 

 quantity of the dry substance is dissolved in strong sulphuric 

 acid in a watch-glass : sections, preferably of fresh material, are 

 then immersed in it for a short time, then washed with water, and 

 mounted in glycerine (see p. 214). 



Hydrochloric Acid. Used, in very small quantity so as to 

 give an acid reaction, with aniline chloride, phloroglucin, or carbolic 

 acid, as a test for lignin. By itself the acid turns lignified cell- 

 walls yellow ; when its action is prolonged, the cell-walls become 

 violet, owing to the presence of various substances such as 

 phloroglucin, coniferin, and pyrocatechin. 



Iodine is one of the most useful reagents : it is prepared for use 

 in various ways. The most important are the following : 



i. Make a strong solution of potassium iodide in distilled water, 

 add to this crystals of iodine and set it aside for some hours, 

 shaking it occasionally : dilute this solution with distilled water 

 to the colour of brown sherry. The reagent may also be prepared 

 by diluting the liquor iodi of the Pharmacopoeia. This is the 

 ordinary iodine solution in common use in the laboratory. 



ii. The alcoholic solution may be prepared by dissolving 

 crystals of iodine in alcohol, and diluting with alcohol to a dark 

 sherry colour ; also by diluting the tinctura iodi of the Pharma- 

 copeia : in the absence of water this solution does not give the 

 blue reaction with starch (p. 46). 



iii. A solution of potassium iodide and iodine in pure glycerine 

 is sometimes used in the treatment of crystalloids (p. 225). 



iv. The solution of iodine in chloral hydrate is used for detec- 

 tion of included starch-grains (see above, Chloral Hydrate). 



v. For the solution in chloride of zinc (Schulze's solution), see 

 above, Chlor-Zinc-Iodine. 



The ordinary solution of iodine (i.) stains proteid substances, 

 and especially the nucleus, brown (p. 27) ; cellulose faintly yellow; 

 cuticularized and lignified walls yellow (p. 39) ; gum purple and 

 starch blue (p. 45). Together with sulphuric acid, iodine colours 



