for the Scented Plants

Our sense of smell

We are born with the ability to detect smells: even one-day old babies pull a face when they are exposed to odours. It is estimated that we can  distinguish up to 10,000 different smells.

Smells are made up of ‘odour particles’ – molecules that are carried  along in the air, so that we can smell some scents from many yards away. When we breathe in through our nose, air passes into a large cavity, called the nasal cavity. In the roof of the cavity is an area called the
olfactory epithelium . The epithelium contains the smell receptors which detect the presence of the molecules. They then send messages to the brain, which interprets them.

In humans the olfactory epithelium is about 5cm2, whereas in cats it’s 25cm2—which is why cats can smell odours which we can’t detect.

Scientists have recently discovered that we have about 1000 genes that code for smell receptors, but that about 600 of these are ‘pseudo-genes’: genes that have lost their function.  Apparently, each of us has a different combination of functioning genes and non-functioning pseudo-genes, which means that everyone has their own individual sense of smell.  So when someone raves about the delicious perfume, and you can only smell a faint whiff of drains, your genes could be to blame!

Our sense of smell also has a psychological factor.  For example, scientists conducted experiments in which people were given two glasses of liquid to sniff: one was colourless, and the other contained a dye.  The majority of people claimed that the coloured liquid smelt more strongly than the colourless liquid, but in fact there wasn’t any difference: the two liquids smelt identical, but because they looked different people thought they smelt different.

It’s possible for your nasal receptors to become ‘fatigued’ by smells: the longer you sniff at a flower, the weaker the scent seems to be. Go away for a couple of minutes and then return—and the smell is as strong as ever.  This can be a problem when you’re selecting scented plants for your garden. Your sense of smell may become fatigued by repeated sniffing, so that you no longer are able to judge how sweet the perfumes are.  The answer is to take a break and come back later.  You’ll find that your sense of smell has been refreshed.

Why do some plants have scented flowers?

It all comes down to reproduction. Most flowering plants rely on insects to transfer their pollen so that seeds can be produced. How do you tell an insect where you are, so that it can find you? One way is to communicate by smell—hence perfumed flowers. Insects such as butterflies are very sensitive to plant scents, and can find a perfumed flower from quite some distance away.

Flowers have to put a lot of energy into producing perfume, so they don’t want to waste it. They produce most scent when the chances are best of their insect pollinators being around to smell it. That means that most  flowers smell most strongly on warm, sunny days, when there’s not much wind – in other words, in conditions that allow the insect pollinators  to be out and about. They also don’t start producing their scent until they’re fully opened, so it’s no good sniffing a half-open bud: you won’t detect much scent until the flower is open.  This is because the flower won’t be ready to be pollinated until it’s fully expanded, so it won’t waste its perfume until it’s absolutely ready to attract a pollinating insect.  And once the flower has been pollinated it doesn’t need to attract more insects, so it will stop producing perfume.  Again, this means if you sniff a faded flower the perfume won’t be very strong.

Some plants are pollinated by night-flying insects such as nocturnal moths. There’s no point in these flowers producing lots of perfume during the day, when those insects aren’t about. Instead, they start to pump out the scent as evening approaches.  Examples include Lonicera caprifolium, the honeysuckle which is native to southern Europe.  Its nectar is situated at the bottom of its long, tube-like flower, which means that only the local night-flying moths have a proboscis (tongue) long enough to reach the sweet reward. 

How is scent produced in flowers?

Scent in flowers is most usually produced in the petals. On the petals’ surface is a layer of cells, called epidermal cells. In these cells various types of fragrant oil are made, commonly known as ‘essential oils’.  The scent of each flower is chemically quite complex: for example, the clove scent of pinks is made up of about 60 different chemicals.

The essential oil is stored in special cells which have had most their contents emptied out of them, turning them into hollow containers. All the while the flower is closed, the oil remains in storage. When the flower opens, the oil is activated: the chemicals form a vapour that floats around the flower, which we smell as scent. Flowers with thick, waxy petals (such as lily of the valley) keep on giving out perfume for longer than flowers with thin petals.

In general, the more colour (pigmentation) the petals have, the less essential oil is produced. White flowers tend to smell the most strongly, followed by pale yellow, pale pink and mauve-pink.

Why do some plants have scented leaves?

Many plants with aromatic leaves grow in hot environments. The oily vapour released by the sun can give protection against the scorching effect of the sunshine by lowering the temperature in the leaves.  In the early part of the 20th century, experiments were carried out into this cooling effect.  Various shrubs were planted against sun-baked brick walls, and the temperature of the wall monitored.  It was discovered that rosemary was able to keep the wall cooler than any other shrub; thyme had the second strongest cooling effect, and third came lavender.

In addition, some oils found in scented leaves are repulsive or even toxic to pests, and so help defend the plant against attack.  Chemical companies are interested in making use of these natural insecticides: for example, it’s been discovered that many insects don’t like the oil in thyme leaves, so there are now flea-collars for cats and dogs on the market made with thyme oil.  Some plants have developed defense mechanisms that not only repel pests, but simultaneously attract other insects that prey on the pests.  And believe it or not, plants can communicate with each other by giving off aromatic vapours: if one plant has been attacked by pests, the other plants around it can detect the chemicals given off by the victim, and they respond by producing the protective chemicals themselves in order to protect themselves from attack.

Aromatic plants, such as herbs, manufacture their aromatic oils in their leaves, and then store the oil in special storage cells.  Some store their oil deep within the leaves, so that you have to crush them quite hard to release the perfume: an example is the bay leaf.  Others, like rosemary, store the oil nearer the surface so that you can smell it by pressing or rubbing the leaf.  Yet others, like thyme, store their oil on the surface so that the scent is released just by warmth.

Leaf scents are more persistent than flower scents. Most flower scents fade when the petals die, but the scents of leaves often become more intense as the leaves dry.  Because some leaf oils are irritants, they can induce an allergic reaction in people, so it’s a good idea not to touch your face or eyes when you’ve handled scented plants.

Smell and memory

Smell and memory are closely linked. A particular smell can conjure up an entire memory, complete with all associated emotions. If we smell something shortly before we have a bad experience, then every time we encounter that smell memories of that unpleasant occurrence will come flooding back.  A customer of ours was admiring our Rosa primula, which has leaves redolent of incense.  The moment she smelt the foliage, she recoiled in horror and said that she couldn’t possibly live with that smell because she had been educated at a convent where she’d been very unhappy.  One sniff of the incense-smelling leaves brought it all back to her.

But it can also work the other way. It has been suggested that if a smell is linked to a positive, healing therapy, then the smell itself can substitute for the actual treatment. Apparently, it works in rats!

Students can make use of this link. Revise a topic while smelling something. Then if you encounter the same smell again you are able to recall the topic more easily. Could be useful in exams—as long as you can take that scent into the exam room with you!

Strange smells

ot all smells are pleasant—even in the plant kingdom. But flowers that seem to us to smell peculiar are intensely attractive to certain insects.

The Dragon Arum, Dracunculus vulgaris, is a good example. It looks like a deep purple arum lily: an extremely handsome plant, with smashing spotted stems and bold divided foliage. But it depends on flies for pollination. And what do flies adore? Rotting meat, so this plant obliges by giving off a pong that fools the flies into thinking they’ve found something delicious. The flies rush in and the flower gets pollinated. (Don’t be put off growing it—just don’t plant it under your bedroom window!)

Have you ever sniffed certain types of Salvia, and realised that the smell reminds you of a school changing room after PE class?  That’s because Salvias contain valeric acid which is also present in human perspiration. Does the foetid odour of Hawthorn, Cotoneaster or Pyracantha flowers make you recoil? That’s because they contain the chemical trimethylamine which occurs in the early stages of putrefaction. Interestingly, Hawthorn flowers manufacture their smell not in their petals but in their stamens, so if you don’t like the smell try the double-flowered forms.  Because they’re stamen-less they don’t have the same pong as the single-flowered types.

One real stinker that you may be familiar with is the Cuckoo Pink (Arum maculatum) which grows wild in this country.  If you touch the spadix, it gives off a smell like stale pee which will cling to your fingers for ages.  This unpleasant smell apparently is heavenly to a type of midge which pollinates the Arum, so to make sure that as many midges as possible get the message the Arum generates heat as the flower opens, sometimes reaching as much as 20 degrees Fahrenheit above the ambient temperature.  This extra warmth helps to volatilise (vapourise) the chemicals which create the smell, ensuring that the pong is quickly and generously distributed.

But scent really is in your own nose. One person’s heavenly perfume is another’s vilest stink. Never take somebody else’s word about a plant: keep on sniffing!