In the foregoing discussion of the components of the volatile oils, we saw that they consist of a variety of compounds which belong to all chemical classes. We cannot expect to find a common history for such varied rubstances. We do observe, however, certain chemical relations between a number of the components. Indeed, it was this similarity that led us to discuss the results of chemical research in terms of four groups, i.e., straight chain hydrocarbons, benzene derivatives, terpenes and miscellaneoos compounds. In view of their structural similarity straight chain hydrocarbons are generally considered as connected with fatty acid metabolism, while benzene and propyl benzene derivatives are connected with carbohydrate metabolism. The group which gives rise to most of the speculation, however, comprises the terpenes.
We have seen that members of this series could conveniently be described as divisible into branched C5 chains. This statement refers to an established fact; but we enter the field of speculation and hypothesis in assuming that such a structure as a C5 chain actually represents the basic unit in the formation of the terpenes in the plants. .
Many terpene investigators have risked guesses as to the nature of this basic unit, but few have tried to support their hypothesis by experiments. One of the oft-mentioned precursors (as we may call them) is isoprene (C6H8), belonging to the group of hemiterpenes. This compound in its turn is postulated to arise from the condensation of acetone or derivatives like dihydroxyacetone and acetaldehyde.64 Through polymerization and addition of isoprene to higher terpenes, terpene homologues can be prepared.66 Among several condensation products dipentene and a bisabolenelike sesquiterpene can be identified66 (Fig. 2.28). When such reactions are carried out under simultaneous hydrogenation or hydration, the reactive ends of the molecules are saturated and further condensation and resinification are thereby largely prevented. Following this principle, Midgley et al.67 carried out the condensation of isoprene under reducing conditions with sodium amalgam and obtained the terpene hydrocarbon 2,6-dimethyloctane. Wagner-Jauregg68 condensed two mols of isoprene in the presence of sulfuric and acetic acids. Under these conditions water is added to the double bonds and goraniol can be isolated from the condensation mixture (Fig. 2.29). Ingenious as these experiments are, they do not furnish proof of the isoprene hypothesis.
Polymerization of Isoprene
 FIG. 2.28. Polymerization of Isoprene.
The same can be said for the hypothetical precursor 3-methylbutenal (Fig. 2.30). This compound would very well satisfy the demands for a reactive precursor. In vitro experiments with 3-methylbutenal, with its conjugated carbonyl group and double bond, clearly demonstrate great reactivity and readiness to react with many other molecules. Fischer69
64 Aschan, "Naphtenverbindungen, Terpene und Campherarten," Walter De Gruyter & Co., Berlin and Leipzig (1029), 127.
66 Forisrhriite Chem. Org. Naturstoffe 3 (1939), 1. Bedeutung der Diensynthese fur Bildung, Aufbau und PMorsehung von Naturstoffen, Diels. M Egloff, "Reactions of Pure Hydrocarbons," Reinhold Publishing Co. (1937), 759.
67 J. Am. Chem. Soc. 51 (1929), 1215; 53 (1931), 203; 54 (1932), 381.
68 Licbigs Ann. 496 (1932), 52.
69 Fischer and Lowenberg, Liebiga Ann. 494 (1932), 263.
Terpene synthesis from 3-Methylbutenal.
 Fia. 2.30. Terpene synthesis from 3-Methylbutenal.
succeeded in this way in building up dehydrocitral which might easily serve as the basic substance for aliphatic, as well as cyclic, terpenes. An added proof would be the synthesis of 3-methylbutenal from acetaldehyde and acetone. Unfortunately, this follows a different addition scheme in vitro; and others have, therefore, suggested the formation of 3-methylbutenal by condensation of acetone with pyruvic acid, followed by decarboxylation. This would also furnish an explanation of the presence of isovaleric acid and pyroterebic acid in the oil of Calotropis procera, where the latter acid occurs esterified with a diterpene alcohol70 (Fig. 2.31). 
Hypothetical Terpene synthesis from Acetone, Acetaldehyde and Acetoacetic acid.
However, Francesconi71 can claim these compounds for his scheme in which isoamyl alcohol has a prominent place. This alcohol is obtained through degradation of carbohydrates, proteins or amino acids like leucine. From leucine, pyroterebic acid and isovaleric acid can be derived with great ease. Huzita72 follows Ostengo in considering isovaleraldehyde to oake a prominent place among the number of proposed precursors. Still another possibility is mentioned by Simpson,73 who couplet acetoacetic acid with 2 mols of acetone to obtain the monocyclic terpenes. The aliphatic terpenes are constructed on paper by linking 3 mols of acetone with one of formaldehyde (Fig. 2.32). Similar hypothetical schemes, using 2 acetone and 2 acetaldchyde molecules, are published by Singleton, 74 and Smedley-Mac-Lean. 76 Available experimental data on these reactions speak against these types of condensation and special factors and conditions have to be postulated in order to account for the directive nature of the plant processes (Fig. 2.32).
Since none of these theories can be definitely rejected or accepted, it is clear that the presence of the branched chain represents a weak foundation on which to build hypotheses on the formation of the terpenes. We also have to admit the possibility that the 5 carbon units into which we candivide the molecules of the terpenes may have their origin in larger units. 
Terpene synthesis according to Emde.
 FIG. 2.33. Terpene synthesis according to Emde.

This suggestion was made by Emde,76 who postulated a physiological synthesis from sugars, through a coupling of levulinic acid-like molecules, followed by loss of CO2 and the addition of smaller fragments of sugar metabolism when necessary (Fig. 2.33).
70 Hesse, in "Organic Chemistry" by Fieser and Fieser (1944), 981.
71 liwistn ital. esscnze profumi 10 (1928), 33.
78 J. Chcm. Soc. Japan 60 (1930), 1025.
73 Perfumery Essential Oil Record 14 (1923), 113.
74 Chemistry Industry (1931), 989.
76 J. Chern. Soc. 99 (1911), 1627.
76 Hclv. Chim. Acta 14 (1921), 881.
The chief value of this clever hypothesis is probably that it points to other ways of constructing branched .molecules. This applies especially to the theory of Hall,77 who attributes the formation of terpenes and benzene derivatives to the condensation and degradation of sugar derivatives. In this way different hypothetical "half molecules" were postulated which are finally combined to give the desired structures. An example of the proposed formations of a terpene precursor is pictured in Fig. 2.34.
Extensive schemes for the derivation of other terpenes and the further synthesis of higher terpenes can hardly contribute to the acceptance of any one of these theories, because once a terpene-likc compound is synthesized on paper it is not difficult to explain the many combinations of terpenes we encounter in nature. Oxydases, reductases, esterases and even special ringclosing enzymes ("Kyklokleiasen" of Tschirch) are therefore welcome instruments in the hands of theorists. In vitro many of the terpenes have been converted one into the other by simple chemical reactions, which take place under physiologically possible conditions. Under the influence of light, air and water, we can expect reactions to take place which we observe in vitro in improperly stored essential oils, i.e., oxidation and polymerization. Free acids, if present, may cause loss of water, cyclization and esterification.
Considering the long storage of these oils in the plant, it is not astonishing that analyses of the oils indicate a gradual change in the expected direction with the maturing of the plant. Experiments on peppermint show an increase in the menthone content with an accompanying decrease in menthol content due to oxidative processes. At the same time, the percentage of compounds other than menthol and menthone increases, indicating a splitting off of water and polymerization.
It is very probable that, in a number of cases, especially in oxidation and reduction reactions, enzymes play, an important role. Ncuborg succeeded in the reduction of citronellal78 to d-citronellol, and of citral 79 to geraniol with yeast. These experiments, extended by Fischer, 80 disclosed certain laws which govern the enzymatic hydrogenation of double bonds between carbon and carbon, and carbon and oxygen. The double bond conjugated with the aldehyde group in citral is slower in its hydrogen uptake than the carbonyl group; and we see, therefore, that the formation of geraniol takes precedence over the formation of citronellal. When geraniol is subjected to further hydrogenation, citronellol is formed, leaving the double bond at C6 untouched.
77 "Relationships in Phytochemistry," Chem. Rev. 20 (1937), 305.
78 Mayer and Neuberg, Biochem. Z. 71 (1915), 174.
79 Neuberg and Kerb, Biochem. Z. 92 (1918), 111.
*Q Fortachrttte Chem. Org. Naturstoffe 3 (1939), 30.

Citronellol produced in this way from optically inactive geraniol is optically active dextrorotatory (specific rotation [αD] =+6) as in citronella oil. No further hydrogenation of the isolated double bond can be effected in this way, and it is interesting to note that in plants also, the hydrogenation has come to a halt at the citronellol stage (Fig. 2.35). 
Enzymatic reductions
 FIG. 2.35. Enzymatic reductions.
Substituents greatly influence the speed of the enzymatic hydrogenations, as seen in the slower hydrogen addition to keto groups, and to double bonds on tertiary carbon atoms. Carvone, main constituent of caraway oil, when subjected to enzymatic treatment, is reduced with difficulty to dihydrocarvone, another constituent of this oil (Fig. 2.36). The absence of the totally hydrogenatcd carvomenthol suggests that similar laws are followed in the production of these terpencs in the plant. These biological reductions can also be followed by studying the excretion products in urine during feeding or injection experiments. While in general, advanced oxidative degradations outweigh hydrogenation processes, a careful analysis of the excretion product shows similar reactions, as in the more simple experiments with yeast or enzyme-systems. Perhaps due to 
precursor. In this way, Francesconi84 explained the simultaneous presence of citral, citronellal, linalool, dipentene, methyl heptenone and acetaidehyde in lemongrass oil. Likewise, Kremers85 correlated the components of American peppermint oil, acetone, acetaldehyde, citral, citronellal, isopulegol, menthol and menthone.

Biological oxidations and reductions of β-Ionone

 Fir.. 2.38. Biological oxidations and reductions of β-Ionone.

The following biogenesis of the two groups of substances found in the oils of American black mint and spearmint was suggested by Kremers. 86 The names of substances actually found in the oils are italicized, while the two reducible groups in the citral molecule are underlined (Fig. 2.39). Structural relationship and frequent occurrence in mint and eucalyptus oils has been noticed by Read87 for the terpenes, piperitone, piperitol, α-phollandrcnc and A4-carene. Piperitone is always accompanied by geranyl acetate, from which many cyclic terpenes can be formed. Read, therefore, has expressed the opinion that the geraniol is a possible intermediate precursor of a number of terpenes. In Eucalyptus macarthuri the chain of reaction apparently stopped at the formation of geraniol, since the oil contains 77 per cent geranylacetate, while in most other species (under different conditions in the plant), more advanced transformations take place.
84 Rivista ital. essenze profumi 10 (1928), 33.
86 J. Bid. Chem. 50 (1922), 31.
86 Ibid.
87 J. Soc. Chem. Ind. 48 (1929), 786.
Biogenesis of Terpenes in Oil of Peppermint and of Spearmint.
converted into many oil components of other species of the same genus. Huzita,89 therefore, considers this linalooliferum plant as the parent species of the genus Orthodon. It is, however, equally well possible that the reactions become blocked at the linalool stage through a mutation process.
Although the tendency has been to explain the formation of the terpene compounds from a Ci precursor like geraniol or citral, it is quite feasible that the condensation of the units takes a different and individual path for a number of terpenes. We are naturally forced to accept this for irwgalarly built compounds such as artemesia ketone and lavandulol, but it might also be equally true for a number of the regularly built terpenes, e.g., pinene. α-Pinene is one of the most frequently occurring oil constituents, 90 and, although the preparation of this ring structure from an aliphatic terpene is unknown, easy roads lead from pinene to a number of mono- and bicyclic compounds, such as terpineol, borneol, camphene, camphor, fenchone, fenchyl alcohol, dipentene, 1,4-cineolc, terpin, pinol, myrtenol, dihydromyrtenol and verbenone (Fig. 2.40). Laboratory experiments may indicate groups of compounds which can easily be converted into each other, 91 but we have always to refer to the composition of the natural oils to give these groups a physiological meaning. It appears likely that in different oils the synthesis of specific compounds (such as limonene) might have taken place in several ways such as by ring opening from pineries or ring closure of citral, geraniol or other cyclic terpenes, or by even direct synthesis.
This individuality of many couplings is further supported by our experience in the higher terpenes, where often, as in abietic acid, one unit is in an irregular position. For an explanation of the different groups of higher terpenes, we have to accept formations from single units, single and double units, doubling of double units, and doubling of triple and quadruple units.
Having reviewed all of these theories, let us summarize the established facts, in order to draw a conservative conclusion regarding the possible synthesis in the plant. We know that :
1. The structural formula of a large number of the compounds in plants can be divided up into branched C& chains.
2. The arrangement of the branched C5 units is in most cases a headto-tail union, but exceptions occur in the monoterpene group, and are common in sesqui-, di- and triterpenes.
3. Ring compounds are easily formed from aliphatic terpenes, whereas the reverse can only be accomplished with difficulty.
4. Oxidation, reduction, shifting of double bonds and polymerization take place readily.
89 J. Chem. Soc. Japan 61 (1940), 424.
90 α-Pinene occurs in 375 oils, according to Ganapathi, Current Sci. 6 (1937), 19.
91 Okuda, J. Chem. Soc. Japan 61 (1940), 161.
Terpen family
FIG. 2.40. Terpen family.
5. The branched C6 unit is distinguishable in the formulas of a number of nonterpenes coupled with nonbranched structures.
6. The terpenes are often accompanied with propyl benzene derivatives and straight chain hydrocarbons.
On the basis of these facts, we may safely conclude that a number of terpenes are formed from a unit which can give rise to one or more branched C5 chains before or after the condensations. It is possible that the (X unit is not the actual structure undergoing condensation, and that more complex compounds are involved, which split off certain groups aftar condensation has taken place. This would include the precursors as described by Hall and Emde, viz., phosphoric acid esters as the sugar precursor, and their degradation products, and protein complexes carrying the condensing structures which release the terpene compounds when formed. The regular head-to-tail union may be predetermined in the compound from which the terpene is formed, or the mechanism of the condensation may be such that this type of union occurs.
The terpenes already formed readily undergo secondary changes, such as reduction, oxidation, esterification and cyclization, and this fact may explain the large variety of derivatives of the same pattern. These familiesof terpenes may have their origin in independently formed key terpenes, such as gcraniol, citral, pinene, etc. Higher terpenes may have been formed through a condensation of lower terpenes of the same or different chain length, whereby quite often derivatives from the regular and symmetrical architecture can be observed. No indications are available that would justify connecting the terpenes directly with other essential oil components, such as straight chain hydrocarbons or propyl benzene derivatives. Although the majority opinion favors a connection through the carbohydrate metabolism in the plant, there is no reason to assume that these products are formed in the same phase of these processes. 92,93 Other essential oil components show structural features strongly suggesting connections with fat and nitrogen metabolism. From chemical evidence we can draw the conclusion that the complexity of the oil composition is caused by excretion or secretion of products formed in many metabolic processes taking place in the plant.
Since the volatile oils are intimately connected with vital processes in the plant, the presence of these specific components has been used also in the determination of the evolutionary status of plant families. 94 A continued, thorough chemical study of the volatile, and especially of the nonvolatile, components will undoubtedly give us a more complete picture of the processes which take place and of the structures which are formed in the metabolic activities of the plant.
92 Simpson, Perfumery Essential Oil Record 14 (1923), 113.
93 Hall, "Relationships in Phytochemistry," Chem. Rev. 20 (1937), 305.
94 McNair, Am. J. Botony 21 (1934), 427. Butt. Torrey Botan. Club 62 (1935), 219.

Although this knowledge must be the basis for any speculation on the mechanism involved, we have to turn our attention again to the living plant itself in order to collect experimental support for our theory of what actually happens. One of the ways in which the plant physiologist tries to solve these problems is to study the cells in which the oils are deposited, and the circumstances under which oil formation takes place
The observation has been made that some of the cells or spaces in plant tissue are filled with oily droplets, difficult to distinguish from fatty oils.
Lysigenous oil sac in ftubus rosaefolius Smith.
These oils can be detected by staining with sudan and osmic acid, and a distinction from fatty oils is best made by taking advantage of the presence of substances with a chemically more active character than the unsaturated hydrocarbons and alcohols, i.e., aldehydes and phenols. For example, droplets containing phenols can sometimes be stained with phloroglucinol hydrochloride. The presence of aldehydes is shown with fuchsin and sodium bisulfite reagents. 95 The oil secretion appears in different cell groups (Illustrations 2.3 and 2.4), and distinctions have been made between external and internal gland cells 96 The external glands are epidermal cells or modifications of these, such as the excretion hairs.
95 Czapek, "Biochemic der Pflanzen," Vol. Ill, 593, Drittc Auflage (1925), Vcrlag G.
Fischer, Jena.

The secretion product is usually accumulated outside the cell between the cuticle and the rest of the cell wall. The cuticle is a thin skin covering the secretions and a slight touch suffices to break this thin piece of skin. Thus, on touching the plant, we observe immediately its well-known scent.
The internal glands are located throughout the plant; they are formed by the deposition of the oils between the walls of the cells. This schism by cells has been called a schizogenous formation. If this is iollowed by dissolution of the surrounding cells, morphologists speak of a schizolysogenous gland formation. 
Tangential section showing oil glands of Washington navel orange fruit.
 ILL. 2.4. Tangential section showing oil glands of Washington navel orange fruit.
Often these intracellular glands have grown to form long canals, coated on the inside with a layer of thin-walled cells. This coating is said to have a double function, viz., the separation of other tissues from the oils and the formation of oils and resins. The secretion forms in the epithelial cells or in the membranes and passes through the cell wall into the interior of the gland. The secretion crosses a mucilagenous material produced by the outer membranes of the secretion cells which has been called the resinogenous layer by Tschirch. This layer does not possess any of the secretory functions ascribed to it, and the designation "resinogenous layer" is in- applicable, at least in the cases of the Umbellifcrac and Rutaccae studied by Gilg and collaborators. 97
96 Haberlandt, "Physiologische Pflanzen Anatomie," 4776, Aufl. 1924, Verlag Engelmann,
Leipzig. Tschirch and Stock, "Die Harze," W35, I, 20 (1933), Verlag Borntriigen,
Studies on the number and distribution of the glands show unequal distribution. The count of the glandular scales in Mentha species shows that the lower surface contains 10-25 scales per sq. mm., the upper surface 1-6 per sq. mm. Dimensions and number of the scales increased near the large vein. 98
If we search the literature99 regarding the exact place of formation of substances like terpenes, we find that a few disputed observations are available, wherein it has been noted that secretion vacuoles suddenly appear in the cell, then increase in number and size, while cytoplasm and nucleus degenerate. These oil globules appear to be surrounded by a membrane. Some observers have seen small droplets of oil, formed in or near the chloroplast, which unite later and form the large oil drops. Others have not observed any oil drops at all in the cells, but found the oil in the membrane layers adjoining the secretion pockets.
Certain observations along these lines seem to point toward the region of photosynthetic activity, where carbon dioxide is reduced and synthesized to carbohydrates. Some support is lent to this thesis by experiments which attempt to establish correlations between oil secretion and known metabolic processes in the plant. Examples of this angle of research are to be found in studies on the effect of climatic and growth conditions on oil content.
A typical example of such investigations is contained in a report on the oil content and composition of Japanese mint (Mentha arvensis) grown in the United States, in which it was established that conditions in southeastern states do not favor the formation of menthol to the same extent as those in the northern and western states. The average differences in large sections of America are of the order of 74.5-81.0 per cent for combined menthol. Data on the individual oils obtained in the different regions show a spread for total menthol of 65.2-88.7 per cent and for combined menthol of 1.7-11.1 per cent. Sievers and Lowman100 rightly stress, therefore, the importance of a critical attitude toward the evaluation of results obtained in such surveys. More reliable evidence is obtained when the handling and oil determinations are carried out under strictly controlled conditions.
Although such statistical experiments are important from a commercial and agricultural point of view, it is difficult to draw any theoretical conclusions as to the physiological effect of climate, soil and other variables.

7 Arch. Pharm. 268 (1930), 7.
98 Hocking and Edwards, J. Am. Pharm. Assocn. 32 (1943), 225.
99 Ttmmann, Ber. deut. pharm. Ges. 18 (1908), 491. Czapek, "Biochemie der Pflanzon,"
III (1925), 585.
100 "Commercial possibilities of Japanese Mint in the United States as a source of natural
menthol," U. S. Dept. Agr. Tech. Butt. 378 (1933), Washington, D. C.\

These data, moreover, give an overall picture of the oil content and composition of young and old leaves, branches and flowers alike. We know, however, that different parts of the plant contain oils which are often of very different chemical composition. As an extreme and almost classical example, the composition of the oil of Ceylon cinnamon might be given. The bark yields oil with a high cinnamic aldehyde content, the leaf oii consists chiefly of eugenol, and the root oil contains a high percentage of camphor. Orange and lemon in flowers and fruit contain oils of different composition, and numerous are the examples where only certain parts of the plant contain oil: oil of iris, valerian and calamus occur only in the roots; sweet birch and cinnamon oils are found in the bark; whereas in the case of santalum album and cedar, the core wood contains the valuable oils.
Better controlled experiments on the influence of climatological conditions, such as sunlight on the oil formation, are found in a series of articles by Charabot and others. 101 Experiments on shaded and unshaded plants indicate that light favors formation of oil. 102' 103 These observations cover a period of several weeks. We possess at least one observation on the daily fluctuations recorded on the oii content of nutmeg sage, the oil yield being 1.5 per cent during the night and in the afternoon only 0.6 per cent. The content of esters is highest toward the evening and least at night. The yield is lower during windy, dry weather. 104
To study oil formation as affected by plant development, it is necessary to select one type of organ and carry out the experiments under rigidly controlled and nonvariable external circumstances. Since this is usually not feasible, the next best results may be obtained in experiments during a stable weather period on fast growing plants, or through the other extreme of very long periods on slow growing plants, thereby averaging the effect of climatic changes.
Although no experimental data exist which will satisfy the most rigid requirements, the second type of experiment is represented by the analysis of oil from the peppermint plant during different stages of growth. Bauer105 analyzed the oils of Mcntha pipcrita at four stages before, and during, bud formation; and during, and after, flowering. His findings are recalculated and summarized in Illustration 2.5, in such a way that the curves represent the percentage of the components relative to the fresh weight of the plant. The different corresponding growth stages are indicated, I, II, III and IV representing the period before budding, during bud formation, flowering stage, and after flowering stage.
101 Charabot and Hubert, Bull. soc. chim. [3] 31 (1904), 402.
102 Lubimcnko and Norvikoff, Butt. Appl. Bot. 7 (1014), 697.
103 Rabak, U. S. DepL Agr., Bur. Plant Ind. Butt. No. 454 (1916).
104 Gaponenkov and Aleshin, /. Applied Chem. U.S.S.R. 8 (1935), 1049.
106 Pharm. Zentralhatte 80 (1939), 353. Relation between the composition of peppermintoil and the vegetative development and variety of the plant.

The percentage of oil increases until flowering, when it either drops or remains constant. This is due chiefly to a decrease in free menthol formation, although the ester menthol continues to increase slowly, but steadily, probably at the cost of the free menthol. The constitution of the oil of a related mint, "Pfalzer mint/' shows the same behavior during development in regard to the increase of ester content. Typical for this mint, however, is the increase in compounds other than the alcohol, probably mcnthone or dehydration products. Similar conclusions can be drawn from the investigations of Charabot106 on leaves of Lavandula, Mentha piperita, Ocimum basilicum, Verbena tryphylla, Artemisia absinthium and Pelargonium. 
Percentages of mint oils and their components at various stages of development
ILL. 2.5. Percentages of mint oils and their components at various stages of development.
lofl Charabot and Laloue, Compt. rend. 147 (1908), 144. Charabot and Gatin, "Le Parfum chez la Plante," Paris (1908). Charabot, "Les Principes Odorants des Vegetaux,"Encycl. Scient., Paris (1912). Charabot, Am. J. Pharm. 85 (1913), 550. Charabot, Compt. rend. 129 (1899), 728; 130 (1900), 257, 518, 923. Charabot, Butt. soc. chim. [3] 23 (1900), 189. Charabot, Ann. chim. phys. [7] 21 (1900), 207. Charabot and H6bert, Compt. rend. 132 (1901), 159; 133 (1901), 390. Charabot and H6bcrt, Butt. soc. chim. [3] 25 (1901), 884, 955. Charabot and Hubert, Compt. rend. 134 (1902), 181; 136 (1903), 1678. Charabot and Laloue, Ibid. 136 (1903), 1467. Charabot and Hubert, Butt. soc. chim. [3] 29 (1903), 838. Charabot and Hubert, Compt. rend. 138 (1904), 380. Charabot and Lalouo, ibid., 1513. Charabot and Hubert, Ann. chim. phys. [8] 1 (1904), 362. Charabot and H6bert, Compt. rend. 139 (1904), 608. Charabot and Laloue, ibid. IZ9 (1904), 928; 140 (1905), 667. Charabot and Herbert, ibid. 141 (1905), 772. Charabot and Laloue, ibid. 144 (1907), 152. Charabot and Laloue, Butt. soc. chim. [4] 1 (1907), 1032. Charabot and Laloue, Compt. rend. 144 (1907), 152, 435. Charabot and Laloue, ibid. 142 (1906), 798. Charabot and
In the later stages of growth the alcohols decrease probably at least partly through ester formation and dehydration to hydrocarbons. This process in turn is followed by oxidative reactions wherein aldehydes and ketones are formed. A decrease in oil content of the leaves during flowering has been observed by Charabot et al. on Verbena tryphylla. 107 In Table 2.
is listed the mg. oil present in different parts of the plant, during the flowering, and after the flowering period. In this period the leaves lost a considerable amount of oil, as compared with other parts of the plant. Analysis of the flower oil showed that the material lost from the flower consisted chiefly of citral. Charabot attributed this decrease in oil content of the leaves in Verbena and Artemisia absinthium1 "* to a consumption of the oil constituents by the flowers, and postulated, therefore, a flow of oil from the leaves to the flowering parts.
When we take into account the way the oils are stored in the plant, and their toxic action when released, this transfer seems unlikely. It is, however, possible that material which otherwise would have contributed to the formation of the oils is used up in the flowering stage, and that the reduced formation of oil is unable to compensate for the constant loss through evaporation. The same explanations can be made for Oharabot's experiment in which it was shown that Mentha pipcrita 10 * and Ocimum basilicum110 plants, after debudding, contain more oil in the leaves than under ordinary circumstances.
Lalouo, Bull. soc. chim. [3] 35 (1906), 912.
Similar results are recorded by Rabak, J. Am. Chem. Soc. 33 (1911), 1242. Nylov,
J. Gov. Bot. Garden Nikita Yalta Crimea 20 (1929), 3. Repts. Schimmel & Co., 1926, 141,
142, 143. Spiridonova, /. Gen. Chem. U.S.S.R. 6 (1936), 1536.
Experiments on salvia seedlings are recorded by Wyslling and Blank, Verh. Schweizer
Naturf. Ges. Locarno (1940), 163.
Data on oil content at different stages recorded by Francesconi, Gazz. chim. Hal. 49, I
(1911), 395. Francesconi and Sernagiotto, Atti accad. Lincei 20, II (1911), 111, 190, 230,
249, 255, 318, 383.
Data on camphor tree recorded by Hood, /. Ind. Eng. Chem. 9 (1917), 552.
107 Charabot and Laloue, Bull. soc. chim. [4] 1 (1907), 640, 1032.
108 Charabot and Lalouo, Compt. rend. 144 (1907), 152, 435.
108 Charabot and Hubert, Bull. soc. chim. [3] 31 (1904), 402.
>" Charabot and H6bert, ibid. [3] 33 (1905), 1121.

 Long-term experiments stretching over two years, and averaging the climatic influences, have been carried out by Charabot and Laloue on Citrus aurantium. From their extensive data, the total oil present in a twig with an attached leaf can be followed through its development. Illustration 2.0 shows clearly the large increase in absolute weight of the oil during the early 
Total oil content in growing leaf and branch of Citrus Aurantium.
 ILL. 2.6. Total oil content in growing leaf and branch of Citrus Aurantium.
period of growth. During the later period the formation in the branches is not even intense enough to compensate for the losses, due to consumption, transportation to other parts111 and evaporation. An increased production of limonene is observed. This is probably formed by the dehydration of the initially present, free and esterified linalool and geraniol. Similar experiments on the oil content at different stages of development were carried out on the oil of bergamot. A tendency in the expected direction was actually observed, i.e., an increase of esters and an increase of terpenes, through the loss of water and through cyclization.
111 Charabot and Laloue, Compt. rend. 142 (1906), 798. Butt. soc. chim. 35 (1906), 912. Hood, J. Ind. Eng. Chem. 8 (1916), 709; 9 (1917), 552. Laloue, BuU. soc. chim. [4] 7 (1910), 1101, 1107.
iG. 2.41. Related Im'vrhc Terpencs 

Composition of oil from growing tips of Eucalyptus cnconfolia.
Relations between Terpcnes in Eucalyptus cneorifolia.
related and belong to the laevorotatory series, constituting additional evidence of their common genesis. A similar relationship for d-phellandrenc has been noted by Berry119 in Phellandria aquatica, viz., d-α- and d-β-phellandrene and the corresponding d-ketone.
Many more observations made on the yield and composition of plants grown under different conditions of soil, climate and treatment, and in different stages of development could be added, but most of these are of such a specific and often experimentally vague nature that they can justify only the general conclusion that the more actively the plant grows, the larger the quantity of oil formed.
To gain a deeper insight into the physiological processes involved in formation of essential oils, we have to limit our experimental subjects to well-defined organs of well-defined species of plants. The experimental work on the composition of the eucalyptus group is a warning that the oils from closely related species may be widely different. Even species indistinguishable by ordinary morphological techniques can be distinguished on the basis of the production of oils of different chemical composition. 120
In many cases, the abnormal behavior is due to hybridization of different apecies. Extensive genetic work has been carried out by Russian workers, and has led to the conclusion that considerable changes in the synthetic; activity of the plants can be observed under the influence of hybridization, so that compounds may appear in the oil which were not present in the parent plants. 121 On the other hand, Mirov in his investigations on the turpentine from the genus Pinus describes a Ponderosa-Jeffrey hybrid which contains terpenes inherited from the Ponderosa parent, and heptane from the Jeffrey parent.122,123
Polyploidy and other types of mutations, such as hetoroploidy and chromosome aberration, may cause changes in the quantity and composition of the oils, as has been demonstrated in Pelargonium roseum. 121
Many factors are, therefore, involved which change the composition of the oils ; and for a successful study of these effects and the solutions of problems of oil formation it is imperative not to add further complications, such
119 Berry, Killen, Macbeth and Swanson, J. Chem. Soc. (1937), 1448.
120 Foote and Matthews, /. Am. Pharm. Assam. 31 (1042), 65. Penfold and Morrison, J. Roy. Soc. N. S. Wales [I] 69 (1935), 111; [II] 71 (1938), 375; [III] 74 (1941), 277.
121 Snegirev, Bull. Appl. Bot., Genetics, Plant Breeding U.S.S.R., Ser. Ill, no. 15 (1936), 245. Nilov, Nesterenko and Mikhel'son, Biokhim. i Fiziol. Drevesnykh i Kustarnykh Yuzhnykh Porod 21, no. 2 (1939), 3. Knishevetskaya, Trudy Gosudarst. Nikitskogo Botan. Soda 21, no. 2 (1939), 29. Mirov, J. Forestry 27 (1929), 13; 30 (1932;, 93; 44 (1946), 13.
123 Kurth, "The Extraneous Components of Wood," "Wood Chemistry," edited by L. E. Wise (1944), 385.
124 Urinson, Bull Appl. Bot., Genetics, Plant Breeding U.S.S.R., Ser. Ill, no. 13 (1936),
as are caused by drying, distilling and harvesting procedures. Storage for a few hours even in the shade may in special cases cause a considerable decrease in the oil content. Russian workers found for nutmeg sage that its volatile oil content decreases 33 per cent after storage for 3 hr., and 55 per  cent after 6 hr. in the shade, while in the sun it decreases 62 per rent after 6 hr. Their conclusion in this case is that the material should be collected at night and immediately distilled.125 The losses in volatile components from intact plants are well known and have been measured quantitatively through micro combustion. The number of excreted products is considerable.
These results126 serve as a warning that external circumstances may easily modify quantity and quality of oils, with the result that changes due to other variables cannot be distinguished. On exposure to air, and especially to sunlight during drying of the plant material in the fields, a considerable amount of volatile oil may be lost by oxidation, polymerization and resinification.
For practical purposes, certain compromises have to be made; nevertheless it should be our goal to choose conditions and experimental material so carefully that reproducibility is assured, and the many factors involved can be changed individually. Only in such a way can we expect to unravel the fate of the plant metabolites secreted as essential oils. Such experiments might well throw light on another intriguing problem, i.e., the function of the essential oil in the plant. A discussion of this subject invites a look at plant metabolism from a more general viewpoint.

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