32 THE VEGETABLE CELL IN GENERAL. 



wholly removed by this acid, even without destroying the proto- 

 plasmic contents ; and this fact has been extensively employed 

 in the examination of the continuity of the protoplasm in con- 

 tiguous cells. 1 



142. The only known solvent from which cellulose can be re- 

 covered without change of composition is Schweizer's reagent, 

 ammoniacal solution of cupric oxide. In this liquid, cellu- 

 lose swells considerably, and slowly disappears. It is thought 

 by some chemists that it does not truly dissolve. From its 

 apparent solution, it can be precipitated in the form of a floccu- 

 lent mass by acids, salts of many kinds, and even by the addi- 



** t/ i/ 



tion of a large amount of water (see 55). 



143. Freshly prepared aqueous and alcoholic solutions of 

 iodine do not color pure cellulose beyond giving a faint yellow- 

 ish tint ; but if the reagents have been kept for some time, par- 

 ticularly in the light, they may impart a blue color. 2 The latter 



1 Unsized, well -bleached linen paper is nearly pure cellulose. If it is dipped 

 in a cold mixture of one volume of water and two volumes of strong sulphuric 

 acid, withdrawn after ten to twenty seconds, and washed thoroughly in water, 

 and finally in dilute ammoniacal water, it becomes much like parchment. This 

 "vegetable parchment" is a suitable membrane for certain experiments in 

 absorption. The acid in this experiment is supposed to convert at least a 

 portion of the cellulose into a substance which closely resembles starch in its 

 chemical reactions, termed amyloid. Parchment paper can be made also by 

 concentrated zinc chloride, and by a few other agents. 



" 2 Mohl (The Vegetable Cell, p. 24, Eng. Trans.) says: " When imbued 

 with iodine, it becomes indigo-blue if wetted with water." In a note on 

 pages 28 and 29, he further says: "My researches shewed me that the in- 

 fluence of sulphuric acid was by no means necessary for the production of the 

 blue colour in membranes which are not strongly incrusted, as in the paren- 

 chvmatous cells of succulent organs, but that iodine and water alone are suffi- 



v O 



cient; while in full-grown and hardened cells sometimes the primary membrane 

 alone, sometimes even a greater or smaller portion of the secondary layers had 

 through the deposition of foreign substances, altogether lost the property of 

 becoming blue on the application of sulphuric acid and iodine, although they 

 were still composed of cellulose, and iodine alone would very readily produce 

 a blue colour in all their membranes after the infiltrated matters had been 

 removed. The means I employed to remove the infiltrated substances were 

 caustic potash and nitric acid. . . . After this treatment, the whole of the 

 layers of all elementary organs are coloured a beautiful blue by iodine even 

 when they offer so great a resistance to the action of sulphuric acid before the 

 treatment with nitric, as is the case in the outer membrane of wood-cells and 

 of vessels, and in the brown cells at the circumference of the vascular bundles 

 in Ferns." 



It is plain that, in the latter cases, the cell-wall had been very powerfully 

 acted on before the application of the iodine, and to this severe preliminary 

 treatment may be ascribed the efficiency of the latter in producing the blue 

 color. 



