Acer oblongum is unique among maples because of the leaf structure and cryptic monoecy. In the present study, an in-depth investigation of reproductive biology has been carried out to study the pollination biology and breeding system of the tree species. The flowers show mixed characteristics of both entomophilous and anemophilous pollination in structure and pollen characteristics, respectively, and can thus be considered as ambophilous (exhibiting traits conducive to both insect and wind pollination). The wind was the primary means of pollination, but exclusion experiments showed that insects (Apis dorsata and syrphid fly) also facilitated pollination by 3–5%. The stigma of hermaphrodite flowers is located 2–3 cm above the level of indehiscent anthers. Interestingly, the insect pollinators hardly came in contact with the pistil; they only brought the pollen close to the stigma and shed it in its vicinity, to be trapped almost like the wind-borne pollen grains on the stigmatic papillae. Anthers of hermaphrodite flowers serve a relict function by attracting insect pollinators, but not contributing to self or cross-pollination. Fruit set following manual geitonogamous and xenogamous pollination showed a difference. The probable reason accounting for low fruit set seems to be the limited number of pollen donors (Staminate type II flowers), high pollen sterility, sparse distribution of individual trees, and geitonogamous pollen. Fruits are dry, indehiscent samaras. Mature fruits do not invariably bear seeds inside since hollow fruits are formed even when ovules inside remain unfertilized.
