vii] OXIDIZING ENZYMES 93 



(d) Dilute a few drops of the filtrate with water in a porcelain dish and add a 

 drop or two of iodine solution. A transient red colour is formed. 



(<e) To a little of the nitrate in a test-tube add a few drops of potassium cyanide 

 solution. A reddish-brown colour is formed which changes to brown but becomes 

 red again on shaking with air. 



In addition to tannic acid, a great many other tannins are known, 

 but their constitution is obscure. 



Expt, 92. To demonstrate that in tannin-containing plants the tannin may be 

 also present in the leaves. Take about two dozen leaves of the Common Oak 

 (Quercus Robur) and pound them in a mortar. Then boil the crushed mass in an 

 evaporating dish with a little water. Filter, and with the filtrate make the tests for 

 tannin. Leaves of other trees also may be used, e.g. the Wig Tree (Rhus Cotinus), 

 Sweet Chestnut (Castanea vulgaris). 



Expt. 93. To demonstrate that tannins may be present in herbaceous as well as 

 u -oody plants. Extract some leaves, as in the last experiment, of Scarlet Geranium 

 (Pelargonium zonale) and test for tannin. 



Expt. 94. To demonstrate that tannins may be present in petals and fruits, in 

 addition to other parts of the plant. Extract and test for tannins as in the last experi- 

 ment, using petals of Pelargonium zonale, Common Paeony (Paeonia ojficinalis) or 

 Hose (any garden variety), inflorescence of Flowering Currant (Ribes sanguineum\ 

 flowers of Horse Chestnut (Aesculus Hippocastanum) or pericarp of Sweet Chestnut 

 (Castanea}. 



THE FLAVONE AND FLAVONOL PIGMENTS. 



These yellow colouring matters are very widely distributed in the 

 higher plants (Shibata, Nagai and Kishida, 26). They are derived from 

 the mother substances, flavone and flavonol, the latter only differing 

 from the former in having the hydrogen in the central 7-pyrone ring 

 substituted by hydroxyl : 



Flavone 



The naturally occurring pigments, however, have additional hydro- 

 gen atoms replaced by hydroxyl groups, that is they are hydroxy- 

 flavones and flavonols, and the various members differ among each other 

 in the number and position of these hydroxyl groups. Some of the 

 members are widely distributed, others less so. Quite often more than 

 one representative is present in a plant. 



