Plant smells: signals, insects and seed dispersal

Volatile molecules can attract insects to an inflorescence or help bats find fruit. But a perceptible smell is not always a signal whose function is known.

A jasmine scent may seem delicate to us, while one of the molecules that make it up becomes decidedly unpleasant at high concentrations. In the article published on 11 November by France Culture with botanist Marc Jeanson, of the National Museum of Natural History, this example is a reminder that our enjoyment is not enough to explain the smells of plants.

The same molecules can guide an insect, help a bat find fruit or play a part in chemistry whose function is still not fully understood. To understand a scent, we need to look at what produces it, how it disperses and what can perceive it.

Smell begins with molecules in motion

A plant does not need a flower to give off a smell. Volatile molecules can leave the tissues of leaves, fruit, wood, bark, seeds or roots, and then disperse through the air. Here, “volatile” means that a substance passes into the air easily enough to be carried to an organism that can perceive it. Crushing a leaf or cutting into bark can therefore release a mixture of compounds rather than a single substance.

The chemical families involved are diverse: terpenes, aromatic compounds, and molecules containing nitrogen or sulphur, among other elements. These terms describe groups of substances, not uniform scents. A smell is often the result of a mixture, whose composition and concentration influence what we perceive. A scent that is pleasant to a human nose therefore does not, on its own, reveal the effect of the mixture on other organisms.

The important distinction is between a signal and the mere presence of a smell. An ecological signal helps trigger a response in another organism. But not every detectable molecule necessarily serves this function. Vetiver roots, for example, contain numerous compounds that contribute to their woody, earthy smell. The biological function of all these substances remains only partially understood. Perceiving this mixture as a scent is not enough to show that it serves as an olfactory message.

Why the smell of carrion attracts insects

The titan arum (Amorphophallus titanum) offers a striking example of an olfactory signal. Its large floral structure is an inflorescence: a cluster of small flowers arranged around a central axis, the spadix, and surrounded by a spathe. It is therefore not a single flower, even though people commonly refer to its flowering.

The plant’s smell is reminiscent of decomposing animal matter. Insects associated with carcasses or excrement may find clues there to an environment in which to lay their eggs. They do not come to help the plant reproduce: they respond to signals that suggest a resource useful to their own life cycle. When they visit the inflorescence, some carry pollen. Pollination is the consequence for the plant, not the insect’s purpose.

The heat of the spadix helps the odorous molecules disperse. The Chicago Botanic Garden says that, when it flowers, the tip reaches a temperature close to that of the human body and that this heat encourages volatilisation, that is, the passage of substances into the air. It also describes the smell intensifying in the evening and at night, then fading in the morning.

The effect can resemble a lure: the smell suggests a resource that insects seek, without being the carcass evoked by the mixture. The plant does not “decide” to deceive them. Over the course of evolution, individuals whose olfactory characteristics encourage certain visits may contribute more to the reproduction of their species. This is the result of selection, not plant intention.

Other smells accompany the movement of seeds

Relationships between plants and animals are not limited to flowers and pollinators. In some tropical pepper plants of the genus Piper, fruit-eating bats find ripe fruit in part by its smell. By eating the fruit, they may then disperse the seeds. This is another stage in the plant’s life cycle: pollination involves the transfer of pollen, while dispersal occurs when seeds are carried away from the plant that produced them.

In this interaction, a characteristic of the fruit therefore matters in its encounter with an animal that may carry its seeds. If the molecules released are detected by animals that eat the fruit, they can contribute to seed dispersal; natural selection may then favour, over generations, the characteristics associated with this interaction.

Our noses cannot tell us what a smell means

A smell that is familiar or pleasant to a person does not necessarily match one that an insect or bat uses as a cue. Conversely, a scent we find repellent may be associated, for some insects, with a place to lay eggs or a resource. Human perception describes our experience of the mixture, not its function in the environment.

To understand what a smell does, we therefore need to look at who can detect it, in what circumstances and with what observable consequence. A plant’s scent may play a role in an interaction, contribute to several functions or remain difficult to interpret. From a molecule that disperses to an animal that detects it, and from pollen to a seed that moves, olfactory ecology links distinct moments in plant life without reducing every scent to a single message.