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 Sept. 2 issue of Nature (Vol. 431), CAS botanists and their German collaborator have discovered a fresh mechanism for self-pollination in the laterally orientated flowers of a Chinese herb 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. It is a deciduous perennial herb which is endemic in the Guangdong and Guangxi provinces 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 seems to be due only to the spreading properties of the oily emulsion and not to gravity. Analysis of the style and stigma under a scanning electron microscope reveals a smooth style and stigma surface, and no channels or grooves. More than 20 days of their observation 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% 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 the extremely shady and humid habitats of C. coenobialis.

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 may be common in species growing in shady, windless and insect-poor habitats.