Led by the nose

Then I have a more outlandish thought. Does the gorse smell me, and know there is a living thing near it? Is it directing its fragrant come-ons my way? This was an outrageously egocentric notion but not out of the question. Natural smells are not just random chemical emissions, they're part of a complex messaging system between plant and plant, animal and plant. Rats emit an airborne chemical signal, a pheromone, when they're afraid which turns on a natural analgesic in other rats in the vicinity to prepare them for pain. When oak leaves are seriously munched by insects they too emit a pheromone which promotes the production of extra tannin in neighbouring trees which makes their leaves more bitter to marauders. Mopane trees in Africa, a favourite food of elephants, do the same and send out messages to other trees when they're being browsed. The elephants are wise to this trick. They eat only a few leaves from each tree and move upwind to new trees. "We can't hear the trees calling to each other," wrote the science writer Colin Tudge, "but the air is abuzz with their consersations nonetheless, conducted in vaporous chemistry."

-- Richard Mabey in The Stinkhorn and the Perfumier, the third of his essays series The Scientist and the Romantic. The reason we know so much about scents we cannot ourselves smell, he explains, is thanks to the electron capture detector invented by James Lovelock in the 1960s:

It's been this instrument which has revealed that fruit flies will respond to as little as one hundred millionth of a gram of a pheromone produced by cassia plants, that lima beans affected by spider mite give off a volatile chemical in minute concentrations that attracts another species of predatory mite that feeds on the original mites. It's helped untangle the extraordinary life cycle of the Large Blue butterfly...

The electron capture detection has also helped us understand the plight of "that gravely threatened creature, the bee":

Honey bees are able to read and interpret chemical cues diffused into the atmosphere over a range forty square kilometres and convey the information back to other bees to their colony. But we now know that the residues of exhaust from cars using lead free petrol react with the odour molecules from flowers, making them indecipherable to bees. This may be one of the causes of the now widespread problem of hive collapse.

Earlier in the essay Mabey discusses scent and memory in his own life, how it unlocks his own "vast structure of recollection" (Proust), not least the associations of various flowers and woodland smells throughout the year. "The puzzle", says Mabey, "is why we're so good at scents despite their having little relevance to our survival and why they're linked to emotion. Scents unlock memories I sometimes didn't know I had":

Smell isn't the oldest sense. The earliest cells must have first acquired an ability to orient themselves in space and respond to warmth, But the identification of food and the necessity of interacting with other organisms entailed the development of this chemical messaging system and we've inherited it. Long before we began to register sense consciously our behaviour was being guided by them. They helped us in finding a mate and bonding with children and tribe, with locating food and avoiding danger, with interpreting the weather and the comings and goings of other creatures. The smell receptors were the foundations of the limbic system, a primitive centre concerned with basic emotions and the recording of sensation, and it was round this that the apparatus of memory began to evolve. Our brains are outgrowth of our noses. No wonder that smells remain the great carriers and triggers of potent memories. They're both processed in the same ancient areas of our brains.

Lewis Thomas had a vision of an entire planet regulated by its smells:

In this immense organism, chemical signs might serve the function of global hormones, keeping balance and symmetry in the operation of various interrelated working parts, informing tissues in the vegatation of the Alps about the state of eels in the Sargasso Sea by long interminable relays on interconnected messages between all kinds of other creatures.