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

The get-out clause

IMAGEBROKER/ALAMY

R KLOOSTER,

MATTHEW

,BLICKWINKEL/ALAMY

:PHOTOGRAPHS

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

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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 With their camouflage of brown bracts, Monotropsis odorata flowers are less vulnerable to herbivores

Hidden agenda Chameleons do it. Cuttlefish do it. Artfully mottled moths do it. And now we know that plants do it too. Botanist Matthew Klooster of Harvard University Herbaria has demonstrated for the first time that plants use camouflage to escape the attentions of their enemies.

Botanists have for decades debated whether plants adopt cryptic colours to blend into their background and avoid the attentions of hungry herbivores, yet no one has produced the necessary proof. Klooster became interested in the camouflage question while studying a group of ericaceous plants called monotropoids. These plants have dispensed with photosynthesis, acquiring nutrients via mycorrhizal fungi, and as a result they consist mostly of out-of-sight roots. When they reproduce, however, they send flowering shoots above ground, where they are vulnerable to herbivores.

‘In most species the shoots are robust, fleshy and conspicuously coloured,’ says Klooster. Monotropsis odorata was different. This rare species grows in wooded parts of the Appalachians in the eastern US and has a protracted reproductive cycle that leaves its flowering stems exposed for four times as long as its close relatives. The plant’s reproductive stems are tiny and the purple stems and pinky-purple flowers are hidden under a dense covering of dry, brown bracts. ‘I was struck by the contrast,’ says Klooster.

cuttings

‘The dried bracts make the diminutive stems almost indiscernible to the human eye. I reasoned that if they’re difficult for people to find, they might also confound herbivores.’

Klooster realised this was the ideal opportunity to test whether what looked like camouflage really was – did the bracts protect the flowering shoots? ‘We could manipulate the colour of the stems and flowers simply by removing the dry bracts. Because they’re dead, there’s no damage to the reproductive structures beneath,’ he explains.

First, Klooster checked that the colour of the bracts was a match for the background leaf litter – not just by eye but by analysing the wavelengths of light they reflect. The analysis confirmed the close resemblance. Next, he had to find out whether blending into the background brought greater success – by enabling the plants to produce more mature seeds. For two years running, Klooster carefully snipped the bracts from some plants, leaving their flowers and stems exposed to view, and left a second group untouched.

The results were clear – plants without bracts suffered almost three times as much damage as those left intact. Plants with bracts also produced up to 20 per cent more mature fruits (American Journal of Botany, vol 96, p2,197). ‘We were able to show that camouflaged bracts do serve a protective role,’ says Klooster. ‘By reducing herbivory they have a direct impact on the plant’s ability to reproduce.’

flowering in the ninth, tenth and fifteenth centuries, but since about 1830 the trend has been for progressively earlier flowering. By the 1990s the average flowering time was earlier than at any time since records began (BiologicalConservation, vol 142, p1,950).

Because cherries have been planted in gardens, parks and other public places, the flowering record is as detailed as it is long. This allowed climate researchers to separate out the effects of rising temperatures because of climate change and the effects of warming caused by increasing urbanisation. Towns and cities, with their brick and concrete landscape, are warmer than open countryside. In 1950s Tokyo, Osaka and Kyoto, cherries flowered at the same time in central, suburban and rural areas. But as these cities grew, flowering times began to diverge. Today, trees in Tokyo bloom as much as a week earlier than those just outside the city. A study of flowering dates in the ancient city of Kyoto attributes a third of the change in flowering time to urban warming and two-thirds to climate change.

Celebrating the annual opening of cherry blossom is a long-held tradition in Japan

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