Smithsonian Gardens Orchid Collection- Dimorphorchis rossii
Currently blooming at the Smithsonian Gardens Greenhouses is a remarkable, large-growing epiphyte endemic to Borneo; Dimorphorchis rossii. As its name suggests, this formidable plant boasts dimorphic flowers, or two different flower forms on the same inflorescence.
The proximal flower morph (typically the first one to three flowers of the inflorescence) is a bright golden yellow and emits a strong fragrance during the day. The distal morph is white with faint pink spots, but has no detectable scent.
Flower dimorphism often occurs as a reproductive strategy (called sexual dimorphism) in plants, with one “male” flower morph containing the pollinia and one “female” flower morph containing the stigma. Dimorphorchis rossii and the other four species in the genus Dimorphorchis are rather unusual because both flower morphs contain male and female reproductive structures.
It is not fully understood why these plants produce bisexual dimorphic flowers, but one hypothesis posits that the strong fragrance and bright color of the yellow flowers serve to attract pollinators for the entire inflorescence, including the non-scented white flowers. This hypothesis is grounded in the idea that producing any sort of pollinator attractant (such as a nectar reward or the chemical compounds for floral fragrance) is energetically costly for the plant, and selection will favor plants that achieve the desired result (pollination) with less energy expenditure.
Another idea is that these dimorphic flowers act as pollinator insurance. The diurnally fragrant yellow flowers may entice a daytime pollinator while the odorless, white flowers farther down the dangling inflorescence (a pollinator syndrome indicative of bat pollination) attract a nocturnal pollinator. It has also been speculated that Dimorphorchis’ pendulant inflorescences, which often reach the ground, are a way for a crawling pollinator, such as a beetle, to travel up the flower spike from the forest floor.
At this point, there is no documented evidence for any of these theories. Dimorphorchis rossii (like most orchids) has not been extensively studied and is a prime example of how much there is still to discover about the ecological relationships surrounding these fascinating and rather mysterious specimens.
-Julie Rotramel, Orchids Collection Contractor