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Learn · Part One — Learning to smell · Lesson 5 of 9

Sticky rice and pandan: two species, one molecule

Two unrelated organisms give the same smell. That tells you a smell does not belong to a plant species.

Crush a pandan leaf. Open a pot of sticky rice. Smell the two one after the other.

You will find they almost coincide, differing only in a thin green layer the fresh leaf adds.

This ought to bother you. The pandan plant and glutinous rice are not closely related. One is a tropical herbaceous plant of the screwpine family, the other a grass of the grass family. They are very far apart on the plant family tree. So why do they smell alike?

The answer

Because both make the same molecule: 2-acetyl-1-pyrroline.

It is a small nitrogen-containing molecule, and the human nose detects it at concentrations below one part per billion. A tiny amount is enough to dominate the whole impression.

Two distant species arriving at the same chemical product is called convergent evolution, familiar at the level of body form — sharks and dolphins are both streamlined although one is a fish and the other a mammal — but less often discussed at the molecular level.

The principle

A smell does not belong to a plant species. It belongs to a molecule.

This is a turning point in how you read every analysis from now on. The list of components matters more than the name of the species. Two different species with the same list will smell alike, whatever plant family they belong to.

And the reverse: two varieties of the same species can give two different lists, and then they smell different. Holy basil is an example — the same genus Ocimum as basil, but one variety is rich in eugenol and smells of clove, while another is rich in methyl chavicol and smells of anise. That is a genetic difference between varieties, not a plant changing direction when it is grown somewhere else.

Three more pairs worth trying

The same principle, three more pieces of evidence.

Clove, cassia leaf and holy basil — three unrelated plants, all containing eugenol, and all three with a clove note.

Cut grass and freshly split bamboo — a small grass and a giant one, the same group of six-carbon green leaf volatiles.

Stone moss, leaf litter and first rain — moss, leaves and soil bacteria, one and the same compound: geosmin.

Smell these three groups and you will stop asking which plant an essential oil comes from, and start asking which compounds it contains.

Further reading for this part

A shared list for the whole of Part One — Learning to smell, not a note for every sentence. We state what each source was used for.

  1. Asifa Majid and Niclas Burenhult, 2014. Odors are expressible in language, as long as you speak the right one A study of the Jahai of the Malay Peninsula, a group with an abstract smell vocabulary. It underpins the central argument of Part One: the gap lies in vocabulary, not in the nose.
  2. Caroline Bushdid et al., 2014. Humans can discriminate more than 1 trillion olfactory stimuli The trillion figure is widely quoted. We cite it together with its critique, because Meister (2015) and Gerkin and Castro (2015) showed problems with how it was extrapolated. It is our example of how an attractive number can spread without anyone checking it.
  3. Những cây thuốc và vị thuốc Việt Nam A Vietnamese-language source for the species names, parts used and sensory descriptions of most of the medicinal plants in the dictionary. Where it and other literature disagree — as with garden mint — we give both.

One thing must be said plainly. We have not read every work above in the original; most of the knowledge comes through secondary literature and reviews. We say so rather than let the list suggest a depth of research we have not reached.

Last updated: September 2026