256 PRACTICAL BOTANY 



The cell-walls do not stain with this fluid, or only slightly. 

 Kleinenberg's haematoxylin stains in a few minutes, whereas the 

 alum solution is much slower in its action. 



Haematoxylin may be used either for fresh material, or for 

 sections which have been previously hardened with alcohol, or 

 with picric or chromic -acid. In the latter case the sections 

 must be washed repeatedly in distilled water to remove every 

 trace of the acid, which, if present, would interfere with the 

 proper action of the hsematoxylin. If the section becomes too 

 deeply stained, as sometimes happens when the alum-haema- 

 toxylin is used, the excess of colouring-matter may be removed 

 by washing with a solution of alum in water. 



Sections stained with alum, or with Kleinenberg's haema- 

 toxylin, are to be mounted in Canada balsam, or Dammar ; 

 those stained with the ammoniacal solution are to be mounted 

 in glycerine. 



Hoffmann's Blue. Used in solution in dilute alcohol slightly 

 acidified with acetic acid : it is a useful reagent, inasmuch as 

 it 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. 



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 sub- 

 stances such as phloroglucin, coniferin, and pyrocatechin. 



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



