April i, 1920] 



NATURE 



139 



Colouring Matters of Plants. 



IN view of the fact that many of Nature's most 

 striking colour effects are produced as the 

 result of harmonious groupings of highly coloured 

 plant life, and that it is to the various plant pig- 

 ments that these fine tints owe their origin, it is 

 not surprising that chemists have striven, from 

 quite early days of the science, to elucidate the 

 chemical structure of these colouring matters, and 

 botanists to discover their relationship to the vital 

 activities of plant life. 



During recent years our knowledge concerning 

 plant pigments has been rapidly and greatly 

 enlarged, and observations have been made that 

 are of great significance to chemist and botanist 

 aUke, whilst the horticultural possibilities which 

 they seem to indicate should be of interest to even 

 the most casual lover of Nature's beauties. 



When referring to plant colouring matters it 

 must be borne in mind that it is necessary to 

 distinguish between the plastid pigments (chloro- 

 phyll, carotin, etc.) and the water-soluble , sap- 

 pigments. The present article will deal only with 

 the latter group — sap-pigments — ^but it must not 

 be imagined that this indicates that progress has 

 not been made in the researches upon plastic 

 pigments ; indeed, much knowledge concerning 

 them has resulted from the extended and intricate 

 work of Willstatter and others. 



The sap-pigments may be divided into two main 

 classes : (i) Derivatives of flavone or of flavonol 

 — sometimes called anthoxanthines — which are 

 pale yellow or colourless when in faintly acid solu- 

 tion, but bright yellow when dissolved in alkalis ; 

 and (ii) the anthocyans, which are red when in 

 acid solution, violet to red-violet when neutral, 

 and of varying tints from dull red, or red-brown, 

 to purple and pure blue when in solution in the 

 form of alkali salts. In both groups the individual 

 pigments differ from each other in the amount of 

 oxygen which they contain in the form of phenolic 

 hydroxyl groups and the arrangement of these 

 groups in the molecule. 



We owe most of our knowledge of the distribu- 

 tion in Nature of the yellow sap-pigments — which 

 usually occur in plant life in chemical combina- 

 tion with various sugars — to the work of A. G. 

 Perkin, whilst the actual synthetic production of 

 a number of these colouring matters by Kostanecki 

 has confirmed our ideas concerning their chemical 

 structure. How widely these pigments are dis- 

 tributed in Nature will be gathered from the fact 

 that members of this group have been isolated 

 from the following sources : Heather, wallflower, 

 clover flowers, cotton flowers, delphinium flowers, 

 onion skins, violas, poplar buds, parsley, etc. 

 Although yellow sap-pigments derived from 

 flavone have been isolated from a large number of 

 plants and flowers, it is quite certain that pig- 

 ments of this group are present in a very much 

 larger number of plants than those from which 

 they have up to the present been isolated. 



When we turn to consider the pigments of the 

 anthocyan class — the purples, reds, and blues of 

 NO. 2631, VOL. 105] 



plant life— the fact of their extremely wide dis- 

 tribution is obvious to everyone. Their presence 

 in petals or leaves is noticeable even where only 

 a small fraction of i per cent, of the pigment 

 exists in the flower. That this is so will be fully 

 realised when the fact is considered that the blue 

 cornflower contains only about 075 per cent, of its 

 dry weight of the blue pigment cyanin. In con- 

 trast with this is the case which has come to light 

 in recent investigations, where as much as 25 per 

 cent, of the flower's dry weight of a yellow sap- 

 pigment was present in a yellow viola, yet this 

 large quantity was completely masked by a mere 

 fraction of i per cent, of a plastid carotin colour 

 that was present in the same flower. 



The great beauty of the anthocyan pigments 

 has given rise to very numerous attempts to obtain 

 an accurate knowledge of their chemical structure 

 and also of their function in plant life. The name 

 "anthocyan" dates back to 1835, and appears to 

 have been introduced by Marquart. Despite the 

 very numerous attempts that were made to isolate 

 these pigments in a pure condition, it was not until 

 1903 that an anthocyan pigment (the colour of the 

 pelargonium) was obtained in a crystalline con- 

 dition by Griffiths. In 1913 Willstatter and 

 Everest described their investigation of the pig- 

 ment of the blue cornflower — which they called 

 cyanin — and laid the foundation of the fuller 

 investigation of the anthocyan pigments that has 

 been developed since that date. It is to Will- 

 statter, to his collaborators, and to Everest that 

 we owe most of our knowledge of these pigments. 

 The identity of a considerable number of the 

 anthocyans has now been established, and pig- 

 ments of this group have been prepared synthetic- 

 ally. Among others, the colouring matters of the 

 cornflower, rose, pelargonium, viola, peony, holly- 

 hock, cherry, and grape have been obtained in a 

 pure condition and investigated. In almost every 

 case these pigments occur in Nature chemically 

 combined with sugars. 



As the result of these chemical investigations 

 the relationship that exists between the yellow 

 sap-pigments derived from flavone and the antho- 

 cyan colouring matters has been made clear. This 

 relationship has been the subject of much study 

 by botanists, particularly by Keeble, Armstrong 

 and Jones, and Wheldale, and it is interesting to 

 note that, whilst botanical work appeared to point 

 to the anthocyan colours being oxidation products 

 of the yellow sap-pigments of the flavone series, 

 chemical investigations have proved that the rela- 

 tionship is the reverse of this — the anthocyans are 

 reduction products of the yellow sap-pigments. 



Very interesting in connection with the function 

 of these sap -pigments in plant life is the fact that, 

 whilst chemical investigations have made it clear 

 that the anthocyan pigments are reduction pro- 

 ducts of the yellow sap-pigments, botanical work 

 strongly points to the conclusion that these very 

 anthocyan pigments occur in plant life in positions 

 that are the seat of oxidising influences. 



