6 MICRO-CHEMICAL REAGENTS. 



If, however, a membrane does not color blue when 

 treated with sulphuric acid and iodine, it is per- 

 meated by other substances, or is to be regarded 

 as changed chemically ; this is true of wood-cells, 

 vessels, cork, and, usually, the root-cap. The 

 substances (lignin, suberin) taken up by the walls 

 in these cases must first be removed by the alter- 

 nate use of acids and alkalies, and finally by 

 washing in alcohol, ether, or chloroform, before 

 the cellulose reaction will appear ; but, as yet, with 

 the single exception of the silicified frustules of 

 diatoms, no cell-wall has been found that is not 

 shown by this treatment to contain cellulose. 



In the study of lichens, chemical reagents play 

 an important part. ' The preparation usually em- 

 ployed, consists of 5 eg. iodine, 20 eg. potassic 

 iodide, and 15 g. distilled water. In thin sec- 

 tions, the hymenium is usually colored blue by this 

 solution, the medullary layer, occasionally, and the 

 gonidial layer, less commonly. A wine color may 

 also appear under this treatment, which shows 

 that these cell-walls have a composition different 

 from those of other plants. 



All living protoplasm 2 is killed by iodine, which 

 is then rapidly imbibed and stains the protoplasm 



1 Deichmann-Branth and Rostrup : Lichenes Daniae, p. 17. De Bary 

 Morphol. und Physiol. der Pilze, Flechten und Myxomyceten. Hof- 

 meister's Handbuch, II., 1866, p. 281. 



2 Sachs: Lehrbuch, 1874. Weiss: Allgem. Bot., 1878, I. Tangl: 

 Protoplasma der Erbse. Sitzungsber. wiener Akad., 1877-8, Bd. 

 LXXVI.-LXXVIII. 



