i8o A TEXT-BOOK OF BOTANY. 



ing principle in madder root. 4. Pyrone derivatives, of which 

 there are several, as gentisein, maclurin, catechin, and rottlerin. 

 being xanthone derivatives ; quercetin, rhannetin, and fisetin, which 

 are flovone derivatives ; and h^ematoxylin and brasilin, being chro- 

 mene derivatives. 5. Isoquinoline, of which berberine is an ex- 

 ample. 6. Benz-pyrrol derivatives ; in this group is indigotin or 

 indigo blue, the coloring principle in Indigofera tinctoria. 



ORIGIN AND FORMATION OF ANTHOCYANIN. At the present 

 time it is very difficult to determine the nature of the chemical 

 processes which underlie the formation of anthocyanin, or the 

 pigment dissolved in the cell-sap and giving the blue, purplish, and 

 reddish color to flowers, fruits, etc. A great many observations 

 have been made on the distribution of anthocyanin, the nature 

 of the constituents with which it is associated in the plant cell, 

 and their relation to various metabolic processes. In order to 

 determine the Mendelian factors for color it is necessary that 

 we have a definite knowledge concerning the nature and formation 

 of this pigment. Of the numerous theories which have been 

 proposed concerning the formation of anthocyanin, that proposed 

 by Muriel Wheldale (Jour. Genetics, 1911, p. 134) seems the most 

 plausible and is as follows : 



(1) The soluble pigments of flowering plants, collectively 

 termed anthocyanin, are oxidation products of colorless chromo- 

 gens of an aromatic nature, which are present in the living tissues 

 in combination with sugar as glucosides. 



(2) The process of formation of the glucoside from chromo- 

 gen and sugar is of the nature of a reversible enzyme action: 



Chromogen + sugar ' < ~^~ glucoside + water. 



(3) The chromogen can only be oxidized to anthocyanin after 

 liberation from the glucoside, and the process of oxidation is 

 carried out by one or more oxidizing enzymes : 



Chromogen + oxygen = anthocyanin. 



(4) From (2) and (3) we may deduce that the amount of free 

 chromogen, and hence the quantity of pigment formed at any 

 time in a tissue, is inversely proportional to the concentration 

 of the sugar and directly proportional to the concentration of 

 glucoside in that tissue. 



(5) The local formation of anthocyanin, which is character- 



