Flowering plants have evolved many different techniques to ensure pollination. For some species, self-pollination is the norm despite the genetic benefits of cross-pollination.

As an evolutionary mechanism, the selfing process allows a plant to propagate, for instance, where pollination agents such as wind or animals are unreliable.

As reported in the September issue of the Nature Magazine, botanists from the Chinese Academy of Sciences (CAS) and their German colleagues have discovered a fresh mechanism for self-pollination in the laterally orientated flowers of a Chinese herb called Caulokaempferia coenobialis, in which a film of pollen is transported from the male anther by an oily emulsion that slides sideways along the flower's style and into the individual's own female stigma.

The plant first aroused the interest of doctoral student Wang Yingqiang and his tutor Zhang Dianxiang, from the CAS South China Botanical Garden during a field survey at the Dinghushan and Nankunshan national natural reserves in 2002.

The plant is a deciduous perennial herb which is endemic in the Guangdong Province and Guangxi Zhuang Autonomous Region of Southern China, where it hangs on rock walls in humid monsoon forests.

After careful studies in the following years, the researchers observed pollination in the angiosperm by pollen that is conveyed in a mobile secreted medium. The lateral flow of the film of pollen along the style and stigma seems to be due only to the spreading of the oily emulsion and not to gravity.

Analysis of the style and stigma of the plant under a scanning electron microscope, reveals a smooth style and stigma surface with no channels or grooves.

Over 20 days observation, the scientists never witnessed any insects visiting the flowers that would have touched the anthers and stigmas, even though each flower can offer up to two microlitres of nectar containing 18 per cent sugar.

Their studies suggest that selfing by virtue of pollen sliding to the stigma may have evolved as a strategy to cope with a scarcity of pollinators in extremely shady and humid habitats.

Scientists say this mode of self-pollination is a new addition to the broad range of genetic and morphological mechanisms that have evolved in flowering plants, and maybe common in species growing in shady, windless and insect-poor habitats.

Editor: Olivia