496 CONSTRUCTIVE AND DESTRUCTIVE METABOLISM 



the most part, while insoluble pigments are contained mainly in chromato- 

 phores. Among the numerous organic substances excreted by fungi and 

 bacteria, pigments are often included. Different pigments may be present 

 in a single cell either mixed together or strictly localized, as for example in 

 a chlorophyllous cell containing red cell-sap. 



Pigment substances are not formed under all circumstances, and they 

 may appear or disappear at certain stages of development. Similarly, 

 the existence of a plant may not be endangered by the non-formation of the 

 normal blue, red, or other pigments of the flower, or by a change in the 

 colouration, as is instanced by the common occurrence of racial and cultural 

 variations. 



The influence of the external conditions upon the production of 

 coloured metabolic products is immediately perceptible, as, for example, 

 when a change of temperature, of illumination, or of the nutritive conditions l , 

 acts as a stimulus inducing altered activities, and either awakening or sup- 

 pressing the formation of coloured metabolic products. A purely physical or 

 chemical action may also be exercised, as, for example, when strong light 

 bleaches and decomposes chlorophyll or other pigments (Sect. 58), or when 

 pigment substances are produced from chromogens by means of the 

 oxidizing agency of hydrogen-peroxide 2 . If when once destroyed the 

 colouration never reappears however long the cell may remain living, it is 

 evident that the pigment is one which is produced only at a certain stage 

 of development. If, however, the colouration returns again, then either 

 the destruction of the pigment must have acted as a stimulus to renewed 

 formation, or else in the living cell fully charged with pigment, the decom- 

 position and reconstruction are continuous and precisely counterbalance 

 one another. Since pigments are metabolic products, it is obvious that 

 the colouration is not produced by any such direct action of light as 

 may be exercised upon many dead substances 3 . 



Most plants are unable to form chlorophyll in darkness (Sect. 58), and whereas 

 red grapes, and the flowers of Tulipa Gesneriana, Pulmonaria officinalis, &c., 

 assume their normal colouration even in the complete absence of light, the floral 



1 [Ewart observed that immersion in sugar solution may induce a formation of red pigment in the 

 tells of certain aquatic plants (The Effects of Tropical Insolation, Ann. of Bot. Vol. XI, p. 477 ; 

 Journ. of Linn. Soc. Vol. XXXI, p. 567), an observation which has since been confirmed and extended 

 by Overton (Jahrb. f. wiss. Bot., 1899, Bd. xxxill, p. 170), who concludes that the formation of red 

 pigment stands in genetic connexion with the presence of sugar in the cell. This may indeed 

 frequently be one of the conditions necessary for the production of a red colouration, but in the 

 majority of cases the action of moderately strong light is also essential. In all such cases the 

 pigment may well have an important biological function and not be merely an accidental by-product 

 of the metabolism of sugar, as Overton suggests.] 



2 Pfeffer, Oxydationsvorgange in lebenden Zellen. 



3 Wiener, Ann. d. Physik u. Chemie, 1895, N. F., Bd. LV, p. 227. 



