68 CELL-CONTENTS AND CELL-WALLS. 



always be tried; the negative reactions with iodine and 

 with aniline sulphate should also be noted when examining 

 tissues. 



N.B. In some cases no blue colour is produced either 

 with chlor- zinc-iodine or with iodine and sulphuric acid; 

 hence the failure of these two reactions must not always 

 be taken as a proof that cellulose is absent, though they 

 are positive proofs of its presence when they are obtained. 

 Sometimes these reactions are not given until the tissues 

 have been treated for some time with potash. Occasion- 

 ally walls turn blue with iodine alone. 



79. Lignified Walls show certain well-defined colour 

 reactions. The chief reagents used are the following : 



(a) Aniline Sulphate (or Chloride) Solution. 



Dissolve aniline sulphate, or chloride, in water, and add a 

 little acid sulphuric or hydrochloric. Dip a wooden 

 match into the solution the wood turns bright yellow, 

 more rapidly on warming it ; if the reaction is not given 

 readily, add more acid. 



(fc) Fhloroglucin Solution. Dissolve phloroglucin 

 powder in alcohol, making a 5 or 10 per cent, solution. 

 Add strong hydrochloric acid until a precipitate just 

 begins to appear ; the solution is then ready for use. The 

 solution may be made up without acid, and the acid 

 applied to the tissue simultaneously with the alcoholic (or 

 aqueous) phloroglucin solution. Dip a wooden match 

 into the solution it turns bright red ; if the colour is not 

 given at once, add acid or apply heat. 



(c) Carbolic-Hydrochloric Acid Mixture. Dissolve 

 some carbolic acid in warm hydrochloric acid; if a pre- 

 cipitate is formed, add enough hydrochloric acid to re- 

 dissolve it, and the mixture is ready for use. Into some 

 of this mixture in a watch-glass lay broken pieces of a 

 wooden match ; on exposure to the light for a short time 

 the wood becomes bright green. 



(d) In addition to these three very characteristic re- 

 actions (which are very readily applied to sections as 



