Star Orchid, or "Darwin's Orchid" (Angraecum sesquipedale) blooming
(1/10/2000) in the Botany Greenhouse.
Darwin suggested that this species and its hawk-moth pollinator provide
one of the most striking examples of how plants and their pollinators can
influence each others' evolution (called coevolution; covered later in
the course). In this case the orchid evolved an incredibly long "nectar
spur", a long tubular extension of the flower that holds the nectar. If
the spur is long, it forces the moths to rub their faces in the pollen
as they reach for the nectar, and the flower is successfully pollinated.
In response to the difficulty of reaching the nectar, their major food
source, the moths evolved longer and longer tongues over time. But if the
moth has a long tongue it can reach the nectar without touching the pollen,
so the orchid has to evolve longer and longer spurs to force the moth to
pollinate it, and so on and so on. This coevolutionary process of orchid
and moth influencing each other reciprocally has been taken to the extreme
in the Madagascar Star Orchid: its nectar spurs can be as long as 11 inches!
When Darwin proposed this scenario, only the orchid had been discovered.
He thought such an impressive spur must have evolved through coevolution
with a pollinator, and therefore there must exist a moth in Madagascar
having an equally impressive long tongue. Everyone thought he must be crazy
until a full 40 years later, when the hawkmoth Xanthophan morgani praedicta
(so named because of Darwin's prediction) was discovered in Madagascar
-- with a tongue that averages a full 10 inches. The Darwin orchid blooms
only once a year, and it's going all-out in the Botany Greenhouse, room
5, right now. If you want a chance to see this world-famous orchid in the
flesh, check it out before the flowers wilt -- you have about a week or
so. The greenhouse is open on weekdays from around 8:30AM to 4:30PM, and
has many more examples of orchids and their diverse adaptations for tricking
insects into pollinating them.
Text by Kimiora Ward Photograph by