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Orchid Has "Active" Sex With Itself -- A Flower First?

Scott Norris
for National Geographic News
June 21, 2006
 
An agile Chinese orchid performs a floral version of sexual
intercourse—with itself.

Researchers say an extension of the male flower part, or anther, turns an upside-down loop to deliver spermlike pollen spores directly into the female cavity.

The anther bends to enter the female chamber from below and is secured in place by a ring structure on its stalk to ensure fertilization.

The flower is the first known plant in which pollination is entirely self-directed, with no outside agents or forces—such as bees or breezes—playing a role.

Biologists observed the elaborate style of reproduction in the bisexual orchid Holcoglossum amesianum, a tree-dwelling plant found in the dry forests of China's southern Yunnan Province (map of China).

LaiQiang Huang of Tsinghua University in Shenzhen and colleagues studied the unusual flower. The team's findings will appear in tomorrow's issue of the journal Nature.

Pollinating Alone

In more conventional plant sex, creatures such as insects or birds transport pollen from one plant to another, resulting in a fertilized embryo or seed.

Most orchids reproduce in this way, and many are known for their elaborate floral structures (photo: South African Disa uniflora orchid) designed to attract specialized insect pollinators.

But pollen may also be transferred from male to female flowers on a single plant, or from male to female parts on a single blossom. In such cases, plants can fertilize themselves.

While less common in orchids, many flowering plants are known to self-fertilize either some or all of the time. Most rely on wind or fluid secretions to move pollen around.

But no other species does it quite like H. amesianum.

"What is strange here is the method," said Mark Johnston, a plant ecologist at Dalhousie University in Halifax, Nova Scotia.

"It's unique that self-pollination is accomplished in so active a manner."

A Practical Solution

Although the orchid produces no nectar and is self-fertilizing, it retains flower features thought to be attractive to insects (related wallpapers: flowers in bloom).

Study co-author Huang says that's a clue that the flower's evolutionary ancestor relied on insect pollination.

Surprisingly, the orchid even retains features of insect-pollinated species that function to prevent accidental self-pollination.

For example, the female cavity faces downward and is kept separate from the male parts by a beaklike projection called a rostellum.

Given such a structure, Huang says, it is practically impossible for fertilization to occur from pollen delivered by the wind, fluid secretions, or the force of gravity.

Wind-assisted pollination is unlikely anyway, he adds, in the still air of the forest interior where the orchid lives.

For H. amesianum, insect pollination is the only conventional alternative. But in their observations of more than 1,900 of the flowers, the biologists observed no insect visits.

"Insects are absent during the dry, flowering season," Huang said.

With conventional pollination methods either unfeasible or unavailable, "self-copulation is the only practical solution."

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