Oncidium sotoanum, Orchid of the Month, September 2017

Oncidium sotoanum (known for a long time as O. ornithorhynchum, but recently subject to one of those annoying "precedence" squabbles that seem to be so important to some) is a moderately large plant with a typical Oncidium growth habit: large flattened pseudobulbs, long thin-textured leaves, and seasonally large branching panicles of small flowers. It is reported to grow in northern Central America in humid tropical rain forests at elevations up to 1500 meters. The species is most famous for its strong sweet fragrance, variably described as vanilla, chocolate, or Fruity Pebbles cereal. I would prefer to say that Fruity Pebbles cereal (one of the many triumphs of American marketing, with the goal of making money by damaging the health and aesthetics of world youth - go marketers!) smells like Oncidium sotoanum.

Specimen Oncidium sotoanum plant grown as an ornamental.

Though the Oncidium sotoanum flower is lovely and smells divine, the species is probably better indirectly known for hybrids in which it dominates, including Oncidium Sharry Baby and Oncidium Twinkle. Both of these hybrids have flower panicles and flowers that resemble O. sotoanum except for color, and both are among the most famous hybrids in the orchid grower's world for their scent.

Presumably this famous scent is intended to attract pollinators, probably male Euglossine bees, which are scent collecting bees often associated with orchids. The scent oils in O. sotoanum are produced by specialized flower cells called elaiophores (from ancient Greek "oil bearer") which form the surface of ridges and pillars near the base of the lip, easily visible in the photo below. This part of the lip is a contrasting yellow or orange color, presumably as a cue to bees in locating the oil. The orchid's purpose, as usual, is to attract and then position these bees so that they pick up and drop off pollinia. Unlike many orchids, this one provides something that the bees want (they in turn use the oils to attract female bees, but that is another story).

Single Oncidium sotoanum (with the old species label) flower, showing the yellow-orange callus where scent oils are produced - notice its proximity to the pollinia, which are not visible but are adjacent to the white patch just above the callus. Photo from IOSPE (Jay Pfahl).

This flower along with many others raises an interesting question: why do so many flowers smell good to humans? Not all do of course, witness the famous corpse flower and many similarly scented flowers, intended to attract carrion flies. However, I think it is fair to say that a randomly chosen scented flower will smell pleasant, or at least intriguing, to most humans. Clearly this has nothing directly to do with us - these flowers have evolved to produce a huge variety of volatile compounds to attract pollinators, usually insects, bats, hummingbirds, and the like, but certainly not humans. It is hard to avoid, but also hard to prove, the conclusion that we have evolved to like the scent of flowers and not the other way around. But why? We don't eat them, we don't defend ourselves with them, indeed they seem altogether useless in every way except aesthetically. Perhaps humans evolved to like the scent so that individuals could pick them and elicit sex by presenting them to their amorous choice, much like Euglossine bees? Certainly that is one of their primary uses in current cultures, although rarely stated so baldly. I suppose this is just barely plausible, though it seems a stretch. More likely, flowers act as cues for the eventual location of fruit, a favorite food of a wide variety of monkeys and apes, including us. Hard to test, but fun to think about. Meantime, if you grow orchids, get this species or one of its hybrids - they smell divine.