RECENT DEVELOPMENTS IN ESSENTIAL OILS 303 (1) Eucalyptus citriodora Hook. The Type or normal form of this species yields an essential oil consisting chiefly of (4-)-citronellal (65 to 85 per cent), but in 1951 (123) it was found that a variant form (var. "A") produces oils in which the major constituents are the alcohol citro- nellol and its esters (citronellate and acetate). The aldehyde content of these latter oils is low, being of the order of 10 per cent. Trees were also discovered that yielded oils with aldehyde contents intermediate be- tween these citronellol-containing oils and the Type oils. In addition, a form was found which contained terpene hydrocarbons as the principal constituents. The presence of guaiol was demonstrated in the oils con- taining 30 to 50 per cent of citronellal (69). Some idea of the frequency of occurrence of these various forms may be gained from data published in 1950 (120). Penfold and his collaborators (117, 127) found that the progeny (192 trees) from a single open-pollinated tree of the low-aldehyde form contained a great variety of oils: 40.1 per cent produced oils cor- responding to the Type (65 to 85 per cent of citronellal), 29.2 per cent be- longed to the intermediate aldehyde group (30 to 50 per cent of aldehydes), 20.8 per cent gave alcohol-ester oils similar to the parent, and 9.8 per cent produced considerable quantities of terpene hydrocarbons. This breeding behavior is similar to that of Leplospermum citratum var. "B" (119, 122) (see below). The observation that terpenic oils were obtained from some of the prog- eny of the variety "A" directed attention to the relationship of Eucalyp- tus citriodora to E. maculata, a very closely related species with a terpenic oil. Oils from E. maculata growing in both New South Wales and Queens- land were examined (95) the New South Wales oils were found to be uniform in nature, consisting of cineole, pinene, dipentene and limonene, sesquiterpenes and sesquiterpene alcohols of the cadinene-cadinol type. On the other hand, the Queensland oils contained no cineole, c•-pinene being the chief constituent. The minor components showed considerable variation. Current unpublished work indicates that E. maculata in New South Wales may be a species different from the Queensland E. maculata, which in turn cannot be differentiated from E. citriodora on morphological evidence alone. The work on the physiological forms of E. citriodora also revealed the surprising fact that no reliable data existed on the nature of the nonalde- hydic portion of the oil of the Type, which has been an article of com- merce for so long. McKern and Spies (93) demonstrated that besides 65 to 85 per cent of citronellal, the Type oil contains chiefly isopulegol, citronellol, the formic, acetic and citronellic acid esters of these alcohols, c•-pinene, •-pinene and isovaleric aidehyde. No evidence could be found for the presence of ge,raniol, cineole, propionic, burytic or valeric acids, re- ported years ago by Ets. A. Chiris (30).

304 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS (2) Melaleuca bracteata F. Muell., one of the "Tea Trees" indigenous to northern New South Wales, and southern Queensland, has also been shown to display physiological forms (121). The normal form or Type produces an oil containing 70 to 90 per cent of methyl eugenol, but indi- vidual trees, indistinguishable morphologically from the Type, and grow- ing in the same district, were found to yield oils containing either methyl isoeugenol or elemicin as major constituents. (3) Backhousia myrtifolia Hooker et Harvey is another plant that ex- ists in physiological forms (116) analogous to those of Melaleuca bracteata, in that major components of similar nature are involved. Backhousia myrtifolia Type yields an oil consisting largely of elemicin, but recent work on Queensland trees has shown the occurrence of forms whose oils may consist largely of either methyl isoeugenol or isoelemicin. In addition, Hellyer and McKern (71) have recently found a third variant, the oc- currence of which was predicted on theoretical grounds here the major component of the oil is methyl eugenol. (4) B•ckhousia citriodora F. Muell. is still another Backhousia species in which a physiological form has been observed (124). Previously it had been thought that the leaf oil of this southern Queensland tree contains from 90 to 95 per cent of citral, but lately trees have been discovered that yielded oils consisting chiefly of (-)-citronellal and (+)-isopulegol, to- gether with small quantities of citronellol and esters. It is of interest that the optical activity (- 14.21ø) of this citronellal is the highest yet recorded for the laevo isomer. The isopulegol is an original component of the freshly distilled oil. (5) Leptospermum citratum Chalk Physiological forms of this plant have been further investigated by Penfold, etal. (122). The study of prog- eny from open-pollinated trees has shown that whereas the Type (70 to 85 per cent of aidehyde) and the vat. "A" (terpenic form) bred true, the offspring of the var. "B" showed segregation into various groups. Forty per cent gave oils of a terpenic nature resembling var. "A", 30 per cent showed aldehyde inheritance (30 to 50 per cent of the oil, i.e., intermediate between Type and var. "B"), 23 per cent resembled the original var. "B" parent and 5 per cent gave oils free from aidehyde, but rich in geraniol and geranyl acetate (alcohol-ester form). Field work has shown that all these classes exist in nature. (6) Melaleuca viridiflora Gaertn. Recently Hellyer and McKern (unpublished data, 1955) examined the essential oils derived from single trees of this "Broad-leafed Tea Tree" occurring on the New South Wales coast. In addition to the trees that contain oils of the linalooLnerolidol form, trees were observed which yield oils consisting of almost pure nero- lidol (about 90 per cent), no linalool being present. Other trees growing side by side with these were found to produce oils rich in the crystalline