22 May 2010

Plant Odours

Hound’s-tongue flowers

Image: Fornax

The accurate identification of wild plants is not always easy, and the botanist must use all his faculties in trying to track down a name for his specimen. One of the most useful yet unreliable senses is that of smell: unreliable because no two people ever seem to react in quite the same way to the same odour. To quote Linnaeus, the founding father of modern systematic botany: “... a scent which is disgusting to a boy is most pleasing to a hysterical woman. A countryman entering a drug-store turns faint with the scent of the perfumes, but recovers when a heap of cow-dung is presented to his nostrils ...”

Besides which, smells usually defy description. The human sense of smell is not very acute, and our language simply has insufficient words for the job.

One of the strangest and most characteristic odours in the plant world is generated by the foliage of the hound’s-tongue, which smells of mice. For me this invariably generates memories of the north Norfolk dunes where I found my first specimens: a commingling of marram, hot sun, and the resinous drift of the air through Corsican pines. Any description that I attempted of the smell of hound’s-tongue would be coloured by this, and meaningless to anyone else.

Nonetheless, a plant-smell, once learned, can be a powerful aid to the memory, even if this knowledge must remain a personal thing and incapable of being accurately transmitted to others.

Animals have no such problems. They have no urge to classify or analyse, and smell is simply one part of the whole which makes up their existence. In some groups smell is just as, if not more, important than vision or hearing, and this is true of many insects, for whom, in the main, plants produce their rich and bewildering variety of scents.

The chemistry of plant odours is immensely complicated. The scent is produced by the oxidation, on exposure to air, of essential oils. The oils are stored in special glands, whether in the flowers or elsewhere in the plant. Glands may also be found in the outer skin of stems and leaves, or the oils may be stored in capsules deeper in the tissues, so that the scent is only released when the plant is crushed.

In flowers, these glands are on the upper surface of the petals, or on the sepals or bracts if these replace the petals. The oils are produced continuously; in the final stage of manufacture, the fluid is left as a mixture of oil and sugar, and it is not until this mixture starts to ferment that the scent is released.

The fragrance given off by a broad bank of wild thyme or a hawthorn hedge in bloom will carry a long way, in insect terms. One function of plant odours is undoubtedly to attract pollinating insects: the brighter and showier the flower, the less likely it is to have a strong perfume. Certain moths and butterflies secrete scents similar to those produced by plants, and in many cases these insects will only visit the flowers which smell like themselves.

On the whole, though, scent is thought to be relatively unimportant in bringing pollinating insects from a distance. It is much more important at close range. In experiments with porcelain flower-models, insects approached in the ordinary way, but would not enter until the models had been brushed with scent from a real flower. So it would seem that the scent modifies the behaviour of the insect in some manner, encouraging it to go through those actions which lead to pollination.

Another function of plant odours is to serve notice on a would-be browser that the plant is distasteful or poisonous. The chemicals adopted here are often trimethylamine and propylamine, which are present in the early stages of putrefaction, or indol and its related compounds, which likewise have a horrible smell. When the depredations of leaf-cutting insects and their larvae are taken into account, this must be vital to survival.

Protection from attack by fungi and bacteria is equally important: it has been estimated that the essential oil of thyme is twelve times more antiseptic than carbolic acid. The oils of lavender and rosemary are just as strong.

Thyme, lavender and rosemary are all members of the Labiatae, the family of plants which includes many of our best-known herbs, such as basil, mint, and marjoram. Labiates tend to be plants of dry, open ground and sunny places, and as such need some protection against excessive water-loss. As part of their armoury they are usually thickly coated in fine hairs, which help to reduce evaporation from the open pores or stomata on the leaves.

The aromatic oils produced by these plants also help to reduce evaporation. The oils are not soluble in water and create a haze round the plant, reducing water-loss but still allowing the exchange of oxygen and carbon dioxide to continue in the leaves. Labiates often grow clustered in dense patches, and it is intriguing to think that they create their own protective shell using nothing more substantial than odour.

But how to explain the smell of hound’s-tongue? To know that it is caused by esters of certain fatty acids does not get us very far. Nor does it help much to know that the same odour occurs in the entirely unrelated lizard orchid, although both plants are pollinated by bees.

Unless plant-nibbling insects are put off by the smell of mice (or goats, as some people describe it), we are left with as curious a puzzle as could be hoped for. Such are the pleasures of botany.

(Introduction to these pieces; see all)

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