Flowering and Fruiting- Osage-orange is dioecious. The simple, green, four-part flowers appear soon after the leaves on the same spurs, opening from April through June, and are wind pollinated. Male flowers are long peduncled axillary racemes 2.5 to 3.8 cm (1 to 1.5 in) long on the terminal leaf spur of the previous season; female flowers are in dense globose heads, axillary to the leaves, about 2.5 cm (1 in) in diameter (2). The female flower in ripening becomes very fleshy, forming a large multiple fruit or syncarp composed of 1-seeded drupelets. The fruit ripens from September through October. The ripe fruit, 7.6 to 15 ern (3 to 6 in) in diameter, yellowish-green, resembles an orange, often weighing more than a kilogram (2.2 lb). Fruits average 23/dkl (80 to the bu) (53). When bruised, the fruit exudes a bitter milky juice which may cause a skin rash and which will blacken the fruit on drying.

Female trees often produce abundant fruit when no male trees exist nearby, but such fruit contains no seeds.

http://www.na.fs.fed.us/spfo/pubs/silvics_manual/volume_2/maclura/pomifera.htm