SPECIAL METHODS. 143 



of water and filtering (cf. Strasburger I, 622). To obtain a 

 sufficiently deep stain, the solution must usually be allowed 

 to act for several hours. It is not washed out by alcohol. 

 [According to Mangin (VIII), pure cellulose is readily 

 stained by many of the azo-colors, as by orseillin BB, 

 crocem and naphtol black in an acid solution, or by Congo- 

 red and benzo-purpurin in an alkaline solution. Several of 

 the dyes recommended heretofore for cellulose walls really 

 stain only the pectic constituents of cell-membranes (cf. 

 292). Such are methylene blue, aniline brown, and chino- 

 lin blue.] 



250. A very deep and permanent staining of the wall is 

 obtained, according to Van Tieghem and Douliot (I), by 

 placing sections, after all cell-contents have been removed 

 by eau de Javelle and caustic potash, and after thorough 

 washing, first in a dilute solution of tannin for one to two 

 minutes and then, as quickly as possible, in a very dilute 

 solution of ferric chloride. They are at once removed from 

 the latter solution and enclosed in glycerine or Canada bal- 

 sam. All the membranes are then stained a deep black. 



For staining the younger membranes of microtome sec- 

 tions, I have lately found Congo-red well adapted. I allow it 

 to act in concentrated aqueous solution, for 24 hours, upon 

 the sections, and then wash them in alcohol and mount in 

 Canada balsam. 



2. Lignified Membranes. 



251. Lignified membranes are distinguished from those 

 of pure cellulose by being insoluble in cuprammonia and by 

 being colored yellow or brown by iodine and sulphuric acid 

 or chloroiodide of zinc. It was formerly generally believed 

 that this difference in chemical relations of lignified walls 

 Is due to the incrustation of the cellulose with a substance 

 richer in carbon, lignin. And in fact lignified membranes 

 give the reactions of pure cellulose after treatment with 

 Schulze's macerating mixture (cf. 9). According to Man- 

 gin (VII), the same thing occurs after treatment with eau de 

 Javelle. 



