pale yellow or colourless unless in the form of alkali salts. In certain cases a fraction of a % of a carrotin colour may cover completely more than 20% of a flavonol colour. The researches of A. G. Perkin and others have resulted in the isolation and examination of a large number of the members of this group, whilst the investigations of Kostanecki have led to their synthetic preparation.
Colours of the second group of sap pigments are called anthocyans, (the glucosides being termed anthocyanins ; the non-glucosides, anthocyanidins). They give rise to the red, purple and blue colours in flowers, and owing to their brilliant effects, have long been the subject of speculation and research. It is only recently, however, that their chemical nature has been disclosed. Willstatter and Everest obtained the pigment of the cornflower in a pure state, and proved that it exists in the flowers as a glucoside. They also showed that by change in the condition of the cell sap one pigment may pro- duce red, purple or blue colours: red in the presence of an acid cell sap, purple if the sap be neutral and blue if it contain the pigment in the form of an alkali salt. Indeed the same pigment (cyanin) has been isolated from red roses and from the blue cornflower. These observations have been extended by Willstatter and Mallison to show how when the presence of pigments of the other groups is allowed for, all variations of flower colours can be explained. Shibati, Shibati and Kasiwagi have recently put forward alternative sug- gestions to account for flower colours, but much of their experi- mental evidence has been shown by Everest to be untrustworthy. Other chemical work by Willstatter and by Everest has elucidated the chemical structure of anthocyan pigments, has shown that they are products of the reduction of the flavonols, and has led to their synthesis. The accompanying formulae show how closely anthocyan pigments are related to the yellow flavonol compounds. (I.) represents kaempferol, a flavonol isolated by A. G. Perkin from a species of larkspur, and (II.) represents pelargonidin, which is the anthocyan pigment present in the flowers of various species of pelargonium.
(ID
H
Cl
H
X)H
H
H
A considerable number of these pigments has now been isolated in a pure condition. It is interesting to note that the honour of hav- ing first prepared crystals of these pigments outside the plants falls to the botanist H. Molisch. Both in the yellow sap pigments and in the anthocyans, the individual pigments only differ from other pig- ments of their own group in the number and position in the molecule of OH, methoxy, or sugar groups.
Before the relationship between flavonol and anthocyan pig- ments had been demonstrated by chemical means, much botanical work had been carried out on this problem, notably by Wheldale and by Keeble, Armstrong and Jones. These investigations con- firmed views expressed many years previously that there was some definite connexion between the yellow sap pigments and the antho- cyans. They also led to the belief that the anthocyan pigments were formed from flavonols. This belief has been greatly strengthened by the proof of the close chemical relationship that exists between the two groups of pigments, and by the work of Everest, Willstatter and Combes, which proved that flavonols could readily be converted into anthocyans. Moreover Everest has shown that in all probability the anthocyan present in the Black Knight viola is accompanied by the flavonol pigment from which it would be produced by reduction.
A number of flavonol compounds has been found to exist in plants in the sugar free condition, but thus far only one anthocyan has been definitely proved to exist in nature in the non-glucoside form that occurring in black grapes.
It is of interest to mention that whilst many of the yellow sap pigments have long been used as mordant colours for commercial dyeing processes, and are so still used to some extent in Europe and more in the East, the beautiful anthocyan pigments also have well marked tinctorial properties and yield fine shades on tannin mor- dants. In the non-glucoside condition they have affinity for metallic mordants, but owing to their lack of fastness in washing their use to any large extent is commercially impracticable.
Beyond the two main groups mentioned above, sap pigments exist which differ in constitution from the members of the main groups. Doubtless the number of these will increase as investiga- tions proceed. An interesting case is that of the pigment of the " Red Pea Gall " recently investigated by Nierenstem.
For reference to the literature see M. W. Onslow, Practical Plant Biochemistry (1920); M. Wheldale, The Antkocyanin Pigments of Plants (1915); A. G. Perkin & A. E. Everest, The Natural Organic Colouring Matters (1918); Shibati, Shibati & Kasiwagi, Jnl. Amer. Chem. Soc., vol. 41, p. 208 (1919); Everest & Hall, Proc. Roy. Soc.
B. vol. 92, p. 150 (1921); Everest, Jnl. Soc. Dyers & Colourists, vol. 34, p. 47 (1920); M. Nierenstein, Jnl. Chem. Soc., vol. 115, p. 1328 (1919). (A. E. Ev.)
IV. Mycology. The recognition of the primary importance of the physiological point of view as compared with the older morphological (or systematic) point of view was prominent as the inspiration of perhaps the most important work in plant pathology during the period 1910-20. In England the brilliant work of Blackman and his school laid the foundations for a scientific knowledge of the physiology of infection by parasitic fungi. These studies have been concerned with Botrytis (Black- man and Welsford, 1916); (Brown, 1915-17); Colletotrichum (Dey, 1919) and Synchytrium endobioticum (Curtis, 1920).
The study of specialized or adaptive parasitism has been followed with fruitful results. The validity of the conception of " bridging species " involving as it does a certain physiological plasticity of the fungus which was accepted by Marshall Ward and Salmon, has been alternately affirmed and denied. Pole-Evans (1911) has asserted that a rust when growing on the susceptible Fi hybrids may thereby become capable of infecting the immune parent used in the cross; Freeman and Johnson (1911) have stated that barley acts as a " bridging species " for biologic forms of Puccinia graminis on other cereals. On the other hand, the number of investigators is increasing who, working with isolated strains of the parasitic fungus under rigidly controlled conditions, have found no evidence for the existence of " bridging hosts." The admirably systematized and patient researches of a band of workers in America, headed by Stak- man, seemed destined to solve this most important question of the constancy, or plasticity, of the " biologic form." The complexity of the problem may be gauged by the fact that a considerable number (at least 22) of " biologic forms " of P. graminis on wheat have now been discovered a fact explaining why the same variety of wheat may be immune in one locality and susceptible in another (Stak- man, Piemeisel, Levine and Leach, 1917, 1919).
Specialized parasitism has been studied also by Barrus (1918), who has found local " biologic forms " of Colletotrichum lindemuthia- num; by Reed (1912-8) and Vavilov (1913) in the Erysiphaceae and by Fischer (1912-7) in the Uredineae. In England Wormald (1919), investigating the " brown rot " (Sclerotinia) diseases, has discovered the existence of two " biologic forms " in S. cinerea, of which one, capable of causing a blossom wilt and canker disease of the apple, is characterized by a more abundant secretion of oxidase. Brierley (1919-20) working with single spore cultures of strains from a " mixed population " of Botrytis cinerea has shown that their phenotypic characters are modifiable but are specific in relation to constant factors.
As illustrating the physiological bent of' many important re- searches that have been made, the following may be mentioned : the relation of soil temperatures to root infection (Jones and Gil- man, 1914-6); (Tisdale, 1916-7); (Edson and Shapovalov, 1920); relations of temperature and humidity to infection by certain fungi (Lauritzen, 1919); (Brooks and Cooley, 1917); re- lations of some rusts to the physiology of their hosts (Mains, 1916) ; chemical changes produced in host tissues (Hawkins, 1916), (Rose, 1915); relations between climate and disease (Stevens, 1917); influence of soil conditions on Thielavia (Johnson, 1919) and Pseu- domonas citri (Lee and Fulton, 1920); physiological studies on spinach showing " Mosaic " disease (True et al., 1918) ; effect of the " black rot " fungus on the chemical composition of the apple (Culpepper et al., 1916).
The bionomics of " potato blight " (Phytophthora infestans) have been much studied. The investigations of Melhus (1915) and Pethybridge (1911) have thrown lighten the nature of the primary seasonal outbreaks; Clinton (1911) made the notable discovery that oospores are formed by the fungus when grown on a certain artificial medium a fact confirmed by Pethybridge and Murphy (1913). Eriksson (1917-8) has stated that " myco- plasm " and non-resting oospores occur in " blight-infected |' potato leaves; since, however, this observer now sees " mycoplasm " in so many directions (rust on cereals, rust on hollyhock (1911), mildew on the gooseberry) independent confirmation of its existence in at least one of the cases is necessary before the " mycoplasm " hypothesis can be accepted. Evidence in marked opposition to Eriksson's statements as to the primary outbreaks of hollyhock rust has been published by Bailey (1920).
From the study of bacterial diseases of plants, no previous period of ten years had seen the collection of so rich a harvest of facts. The indefatigable work of Erwin F. Smith constitutes by itself an in- valuable library of exact information. In his researches (1912-20) with Bacterium tumefaciens, the organism causing " Crown gall ' in plants, proof has teen obtained that the gall formed at the point of infection gives rise to tumour strands which push their way through the surrounding tissues and develop secondary and tertiary growths, analogous to what is found in animal sarcoma, carcinoma and embryoma. Numerous other workers, e.g. Morse (1917) in the United States, Doidge (1915-9) in South Africa, and Paine
