140 



NATURE 



[April i, 1920 



It has been noticed by many who have investi- 

 gated the anthocyan pigments that there is always 

 at least a trace of yellow sap-pigment present 

 alongside the red, purple, or blue of the antho- 

 cyan. From this has arisen the belief that the 

 anthocyans are produced in Nature via the yellow 

 sap-pigments, and recent work has shown that 

 there is very considerable ground for thinking 

 that this belief may prove to be correct. 



To even the most uninitiated, the chemical 

 formulae representing a typical anthocyan \_e.g. 

 delphinidin (I.)] and the corresponding yellow 

 sap-pigment [myricetin (II.)] make it obvious 

 that a relationship exists between them— 



HO 



I \ ■ 



OH 

 ^ OH 

 Lll 



HO 



-OH 



(I. 



Naturally, to the horticulturist the interrela- 

 tionship of the various sap-pigments to one 

 another is of great interest; also the effect of 

 these colours upon the tints produced by the 

 plastid pigments that occur with them in plants 

 and flowers. The proof, by chemical investiga- 

 tion, that the blue cornflower owes its colour to 

 the same pigment as the red rose is of the greatest 

 interest, for does it not raise hopes of success in 

 the endeavour to produce a pure blue rose? In 

 the rose the colour is red because the sap is acid, 

 whereas the cell-sap in the cornflower is in such 

 a condition that the pigment can take up enough 

 alkali to form its blue alkali salt. Can the latter 

 condition be reproduced in the rose? 



It is often erroneously stated that the yellow 

 sap-pigments are responsible for the yellow tints 

 in flowers and berries, but in reality the bright 

 yellows are almost exclusively due to plastid 

 colours related to carotin, whilst the orange and 

 brown tints are produced by combinations of these 

 colours with those produced by pigments of the 

 anthocyan group. In some few instances, how- 

 ever, it is probable that sap-pigments give rise to 

 fairly strong yellows, but, in general, members 

 of this class of compound produce pale yellow 

 tints such as the colour of the primrose, or are 

 present in an almost colourless condition in the 

 acid cell-sap of white- or cream-coloured flowers. 

 It is exceedingly difficult, even for one who has 

 studied the pigments minutely, to be certain by 

 mere observation which of the anthocyan pigments 

 is present in any flower that may be examined. 

 Chemical work has shown that plants of the same 

 botanical group may produce different pigments, 

 and, indeed, that more than one anthocyan, or 

 yellow sap-pigment, may be present in the same 

 flower. 



Very naturally the clothing of Nature in such 

 beautiful tints, as the result of the presence of 

 these colours, led to the desire on the part of 

 NO. 2631, VOL. 105] 



man to use them for the colouring of garments 

 and other textile materials. Many of the members 

 of the yellow sap-pigments are capable of indus- 

 trial use as mordant dyes, and were largely so 

 used before the synthetic colours became available. 

 Some of them — e.g. fustic — are still employed 

 in considerable quantities even in European 

 countries. In the East quite a number remain 

 in use. Concerning the dyeing properties of the 

 anthocyan pigments, much doubt seems to 

 have existed, but it appears certain that in 

 1850-60 the colour of the hollyhock was largely 

 used in Bavaria for dyeing purposes. Quite 

 recently these colours have been more fully investi- 

 gated in respect of their dyeing properties, and 

 it has been found not only that they dye wool, but 

 also that they are capable of giving very fine 

 shades when used on cotton with a tanning mor- 

 dant. Although they have considerable tinctorial 

 power, and the dyeings produced by them are fast 

 to light, they do not stand washing sufficiently to 

 make It possible for them to hold their own 

 against synthetic colours. 



Apart from the two main groups of sap-pig- 

 ments, with which the above remarks have been 

 concerned, there are quite a considerable number 

 of coloured compounds that exist in plants in some 

 soluble form— usually as glucosides. It should be 

 noted that, whilst flavone or flavonol derivatives 

 are very widely distributed, and the anthocyan 

 pigments almost equally so, the remaining colours 

 are much more restricted in their distribution. 

 What y6\e the flavones, either alone or accom- 

 panied by anthocyans, play in plant life, other 

 than that of decoration, has not yet been dis- 

 covered. Wide distribution is no indication of 

 commercial importance as regards plant colouring 

 matters, and some colours that are by no means 

 widely distributed are of considerable importance. 

 Furthermore, the question of plant colouring 

 matters does not end with the considera- 

 tion of those colours that exist ready-formed in 

 the plant. Indigo, the most important of all plant 

 colouring matters, exists in plant life as a soluble, 

 colourless glucoside called indican, which produces 

 indigo only when it loses the sugar with which 

 it is chemically combined, and is oxidised by con- 

 tact with air or other oxidising medium. The 

 archil or cudbear group constitutes another class 

 of colours that were formerly of commercial 

 importance, and are produced from soluble 

 colourless products present in various lich.ms. 

 In conclusion, the important dye alizarine 

 should not be omitted ; this product was formerly 

 obtained exclusively from plant sources — chiefly 

 madder-root, in which it occurs partly as the 

 glucoside ruberythric acid — whereas almost all the 

 alizarine that is now used is prepared synthetically 

 from the coal-tar product anthracene. It would 

 appear that the time is not far distant when all 

 plant pigments that are used for technical purposes 

 will be displaced by synthetic products, but the 

 recent shortage of synthetic dyes has certainly 

 somewhat prolonged the commercial life of the 

 various natural colouring matters. 



