Genus Asarum L. (Aristolochiaceae)
contains about 90 species, mainly distributed in SE Asia. 39 species occur in China in which one is endemic to Hong Kong - Asarum hongkongense, a critically endangered species.
Flowers of Asarum are close to the ground and have strange shape. It is commonly believed that flowers of Asarum are pollinated by insects due to the traits of the flowers like the calyx surface ornamentation, tiny glands on the inner calyx surface and caudate sepal lobe extensions. Unlike most flowers of Aristolochia, flowers of Asarum give no bad odor which suggests Asarum is not imitating animal corpse that attract flies to pollinate. An old study suggested that the flowers resemble mushrooms that attract fungal gnats to lay eggs inside the flowers which may pollinate the flowers. However, more recent studies demonstrate that Asarum produces autogamous flowers which undergo self-pollination. The putative insect-attracting floral characters are therefore primitive which maybe derived from an insect-pollinating ancestor.
Flowers of Asarum are protogynous. The stigmas are receptive at the beginning of anthesis while anthers lie horizontally at the bottom of the flowers so that stigmas and anthers are spatially separated. In the later stages of the anthesis, the stamen filaments become erect (See the second picture) and the anthers dehisced to release pollen grains which promote intrafloral pollination. However, no detailed phenological study has been carried out for A. hongkongense so far, it is still not sure if its pollination mechanism is like such case.