PIGMENTS 



495 



which hive the power of fixing oxygen, they have for the most part solely 

 a biological significance, being commonly used for attractive purposes in floral 

 organs, &c., and in leaves and young stems serving as a protection against 

 excessive insolation. The absorption of certain rays necessarily produces a 

 corresponding rise of temperature, by means of which the rate of transpiration 

 may be increased and the general vital activity accelerated so long as the 

 optimal temperature is not surpassed *. In other cases the colour is simply 

 an accidental property of certain products of metabolism, although these 

 have usually no marked colouration. Thus it is difficult to see of what 

 importance the colouration, that is, the more marked absorption of certain 

 rays and reflection of others, can be in duramen, in fungi growing in dark- 

 ness, or in those pigment-bacteria which can occlude oxygen. 



The red and blue pigments dissolved in the cell-sap, such as anthocyan 

 and erythrophyll,seem to be tannins or compounds allied to phenols, whereas 

 the fatty lipochrome and chlorophyll pigments, as well as the ptomaine- 

 like bacterial product pyocyanin, are of quite different chemical con- 

 stitution. Aniline dyes are formed by a few bacteria, and algae possess 

 in phycoerythrin and phycocyanin coloured and crystalloid proteid sub- 

 stances (Sect. 60) 2 . In some cases a pigment (indigo, alizarin) is formed 

 from a pre-existent chromogen by decomposition or oxidation (Sects. 84, 87), 

 so that the last stage in the production of pigment is rendered directly 

 evident. Tannins and other compounds allied to phenols may perhaps 

 also serve as the precursors of pigment substances, which, however, are 

 certainly often formed from other materials, and may often occur in plants 

 which contain no tannin 3 . 



In the higher plants soluble pigments are dissolved in the cell- sap for 



1 Cf. Ludwig, Biol. d. Pflanzen, 1895, and also Sect. 62. On the importance of the warming 

 effect, cf. Kny, Zur physiol. Bedeutung d. Anthocyans, 1892 (Atti del Congresso Botanico, 1892) ; 

 Stahl, Ann. d. Jard. hot. de Buitenzorg, 1896, T. xni, p. 137. According to Pick, the absorption 

 of certain inimical rays permits the translocation of starch to proceed more actively (Bot. Centralbl., 

 1893, Bd. xvi, p. 346), but this might be simply due to the heating effect. In certain tropical 

 plants which grow in damp and shady situations, the dye appears to be mainly of importance as an 

 aid to transpiration, especially when present only on the under surfaces of the leaves. In more ex- 

 posed situations, it seems to act almost solely as a protection to young, sensitive, and especially to 

 chlorophyllous organs, or ones in which synthetic metabolism is active (Kwart, Annals of Botany, 

 1897, Vol. XI, p. 460). In darkness little or no pigment is formed, and it is interesting to notice that 

 the absorption-spectrum of erythrophyll is to a large extent complementary to that of chlorophyll 

 (Engelmann, Bot. Zeitung, 1887, p. 433; N. J. C. Miiller, ibid., 1888, Bd. XX, p. 78). 



2 Cf. Hansen, Farbstoffe d. Bliithen u. Friichte, 1884; \Vigand, Bot. Hefte, 1885, p. 218; 

 Zimmermann, Mikrotechnik, 1892, p. 97; L. Miiller, Vergl. Anat. d. Blumenblatter, 1893, p. 230; 

 Weigert, Bot. Centralbl., 1896, Bd. LXVI, p. 353, and the literature here given. On bacteria and fungi : 

 Zopf, Pilze, 1890, p. 143 ; Fliigge, Mikroorganismen, 1896, 3. Aufl., pp. 175, 487 ; Lafar, Technische 

 Mykologie, 1897, p. 125. On lichens: Bachmann, Jahrb. f. wiss. Bot., 1890, Bd. XXI, p. I ; Zopf, 

 Ann. d. Chemie, 1897, Bd. ccxcv, pp. 222, 295. 



3 Cf. Wigand, I.e.; Pfeffer, Unters. a. d. Bot. Int-t. z. Tubingen , 1886, Bd. II, p. 218; G. Kraus, 

 Grundl. z. Physiol. d. Gerbstoffes, 1889, p. 31. 



