-114 BOTANICAL MICROTECHNIQUE. 



the hyphae. These crystals are colored brown only at the 

 surface of the fungus, those in its interior being at most 

 pale gray or yellowish. In the air the colorless crystals 

 gradually assume a brown color. According to Thorner, 

 they represent a hydroquinone-like body which gradually 

 passes over into the corresponding quinone. For the recog- 

 nition of the quinone, Bachmann (II, 7) recommends strongly 

 dilute caustic potash or soda, which instantly dissolves it with 

 a greenish-brown color. 



9. Tannins. 



198. All those substances which give a blue-black or green 

 color with iron salts are commonly designated as tannic 

 acids or tannins. There belong here, of the better known 

 compounds, especially : 



Pyrocatechin, C 6 H 4 (OH) 2 ; 



Pyrogallic acid, C 6 H 3 (OH) 3 ; 



Protocatechuic acid, C 6 H 3 .(OH) 2 .COOH ; 



Gallic acid, C 6 H 2 .(OH) 3 .COOH ; 



Gallo-tannic acid, C U H 10 O 7 (= digallic acid ?) ; and besides 

 these there are many other compounds whose constitution 

 is not yet certainly determined (cf. Beilstein, III, 431 fL). 

 We have no trustworthy methods for the certain micro- 

 chemical distinction of these substances, although this is 

 the more to be desired since we have certainly to do with 

 very different substances and, as Reinitzer (I) has lately 

 shown, it is very hazardous to assume a common physio- 

 logical function for this whole group of compounds. 



But a detailed account of the methods used for the recog- 

 nition of the whole group of tannins seems to be demanded 

 by their wide distribution in plants. The following reac- 

 tions have been made use of in their study : 



<r. Iron Salts. 



199. Of the iron salts, an aqueous solution of ferric chlo- 

 ride was formerly chiefly used ; but it has the disadvantage 

 that it has nearly always an acid reaction, and when used in 



