Australian Geographic — May-June 2017

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92 Australian Geographic

and partly as a result of that perception, they are now found

widely, from California and China to India and Algeria.

Understanding the chemical source and purpose of

aromas can help show us how ecosystems work. For exam-

ple, the essential oils that give some plants strong odours

repel bacteria and fungi, while also deterring grazing

mammals and insects. So it’s not surprising that eucalyp-

tus oil serves well in toilet cleansers and tea-tree oil is a

valued germicide.

Aboriginal healers similarly employed fragrant plants,

especially paperbark, emu bush (Eremophila sp.) and north-

ern sandalwood (Santalum lanceolatum). Even birds use them:

eagles and other birds of prey often place eucalypt sprigs

in their nests, apparently for sanitation.

Many plants produce chemical defences that are stronger

than essential oils. The most potent, including alkaloids,

contain nitrogen, an element that is scarce in Australia’s

infertile soils. Large numbers of our plants rely instead on

nitrogen-free defences, including aromatic compounds

that typically contain only the three freely available con-

stituents of carbon, hydrogen and oxygen.

These compounds are produced in mixtures that give

each plant its own protective bouquet. Because of this, the

fragrance of the Aussie bush is a statement about plants on

poor soils defending themselves with cheap ingredients.

The main oil in eucalypts, known as cineole, or euca-

lyptol, is the source of a liniment smell. Because it is pro-

duced by many native plants, including paperbark, mint

bush and the liniment tree (Asteromyrtus symphyocarpa), it

provides the signature smell of the Australian bush. We’ve

employed it in cough lozenges, wound sprays, antiseptics,

grease removers and cigarettes. German scientists recently

found that it benefits asthma sufferers, which would not

have surprised those 19th-century doctors who prescribed

eucalypt cigarettes.

Hold the leaf of an aromatic plant to the light and, if

it’s not too thick, you can usually see the translucent dots

of oil glands that release the aromas, sparkling like stars at

night (a magnifying glass helps). On some plants, they are

so large the leaves look warty. Oil glands can make up to

20 per cent of dry leaf weight, and often leave fingers sticky

when foliage is crushed.

Aromas are handy for botanists, who, by crushing and

sniffing, can tell if an unfamiliar heathland shrub is a citrus

or eucalypt relative and whether a rainforest tree is one

Backhousia species or another. One orchid, the hooded

caladenia (Caladenia cucullata), distinguishes itself from the

look-alike musky caladenia (C. gracilis) by having flowers

The fragrance of the Aussie bush

is a statement about plants on

poor soils defending themselves

with cheap ingredients.

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Colin (at left)

and Tobias

Ferguson sniff

weeping tea-tree, an

aromatic plant they

use to combat colds

and blocked noses.

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Native to

south-eastern

Australia, the blotchy

mint bush is a fragrant

member of the mint

family, along with true

mints and other herbs.

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PHOTO CREDITS: TIM LAMAN. SCIENTIFIC NAMES, FROM LEFT:

Melaleuca quinquenervia;

Prostanthera walteri