II. THE ORIGIN OF ESSENTIAL OILS
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.
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.
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).
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.
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).
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.
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.
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.
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.
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.
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,
Berlin.
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.
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,
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
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.
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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