Insects get trapped in the flowers of Aristolochia, but are released once they’re covered in pollen

The get-out clause

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R KLOOSTER,

MATTHEW

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One of the most fascinating special relationships between plants and their pollinators is that of jailer and captive, where flowers trap insects to ensure pollination. The classic examples are Aristolochia species, with their distinctive tube-shaped flowers that lure in insects and hold them prisoner until they’re loaded up with pollen.

the male anthers ripen and release pollen. Pollinating insects, thought to be small flies, enter the flowers during the female phase, travel to the utricle, and remain there until the anthers shower them with pollen.

Oelschlägel and colleagues examined flowers of six Mediterranean species of Aristolochia with a scanning electron microscope and then performed intricate experiments to test how effective hairs were as traps (New Phytologist, vol 184, p988).

Trichomes are narrow, cone-shaped hairs that are attached to the tube wall by a narrow stalk. The offset position of the stalk ensures that the hair bends further in the downward

In 1891, German botanist Carl Correns suggested that the key to the success of Aristolochia species in trapping insects is the hairs, or trichomes, lining their flower tube. These, he suggested, eased entry to the flower but hindered attempts to escape. More than a century later, Birgit Oelschlägel of Dresden University of Technology has tested Correns’ idea and shown he was right.

Aristolochia need to cross-pollinate, so pollen-dusted flies must escape the trap

The reproductive organs of Aristolochia lie in a small chamber, or utricle, at the base of a long flower tube. The female stigmas mature first, withering a day or two later when direction than upwards, encouraging visiting insects to move in one direction – towards the utricle. Oelschlägel found that the hairs are also covered with small wax rods that make an insect’s feet slip and slide, propelling them downwards.

To test the force necessary to pass in and out of the flower tube, the team

14 l KEW Spring 2010

made fake flies by coating small glass spheres with wax and rosin. They attached the ‘flies’ to threads and pulled them through flower tubes while measuring the flower’s resistance to their passage. In reality, a fly wouldn’t be spherical, and other forces would also come into play,

but this simulation shows that any insect trying to push its way out of the tube faces a formidable obstacle. In the case of Aristolochia baetica, there was a 75-fold difference in the force required to get in and to get out. ‘Flies don’t need to exert large forces to enter the trap, but to escape during the female flowering stage a large effort is required that’s probably not achievable by insects this size,’ says Oelschlägel.

Aristolochia, however, need to cross-pollinate, so pollen-dusted flies must escape and visit other flowers with receptive stigmas. ‘That means the flower must disengage the trap,’ says Oelschlägel. Her electron microscopic examination revealed that after flowers have produced pollen the trichomes are transformed. ‘The side walls of each cell collapse and are folded like an accordion,’ explains Oelschlägel. The result is a shrivelling and shortening of the hairs. That shrinkage opens up a passage to the outside world, while the newly crinkled surfaces provide firmer footholds for the escaping flies.

Blossom festival tracks long-term changes

Each spring, Japanese people eagerly await Hanami, the cherry blossom festival, and enjoy a day of picnicking, partying and a stroll among the flowering cherries. Climate scientists have also begun to appreciate Hanami, for such is Japan’s obsession with cherry blossom that there are flowering records for the last 1,200 years.

In recent years, ecologists and climate scientists have taken a close interest in the dates when trees first leaf, flowers open or birds return from their wintering grounds – all markers of the arrival of spring. For most of these natural events, records go back less than a century. In Japan, however, records go back to the ninth century. These show that cherry blossom time can vary from late March to early May. There were periods of early