Maple tree fruit are known as keys, and they spin and float on the air as they fall from the tree in late spring. There are 148 species of maple tree found in the Northern Hemisphere, including 90 native and introduced species in the United States. Maple trees and shrubs are of the genus Aceraceae, whose name means "sharp" in Latin. The leaves have from three to nine sharply pointed, serrated lobes. Maple tree fruit, called samara, carry the seeds from which the trees propagate.
Maple tree samaras have a single winged formation which causes them to twirl and spin as they leave the tree and fall to the ground. Other names for maple tree fruit include maple keys, whirligigs and spinning jenny. A spinning jenny is a frame used to manufacture cloth in northern England in the eighteenth century. Maple tree samaras contain a single seed on one side of the wing-shaped key.
A maple tree key is a flattened, fibrous, papery tissue that encloses the single maple seed. Also called an achene, other plants such as strawberry and buttercup also reproduce by this method. The archene contains the seed but does not open upon maturity. The single winged shape of the maple key enables it to be carried on wind and distributed to a wide area. Maple trees develop flowers which are pollinated to produce samara.
Maple flowers appear in late winter or early spring. Flower colours include red, green, yellow or orange, depending on the species. They form in drooping clusters 1 to 2 1/2 inches long. Individual flowers are tiny but the overall effect on the tree is of bright colour. Maples have both male and female flowers on each tree, which provide pollen and nectar for bees.
Flowers mature in mid-spring and begin to form the single winged keys which contain the seed. Keys mature in a few weeks to six months after flowering and begin to spin and fall to the ground for germination. Germination may begin immediately or delay until the following spring. Seedlings sprout easily in shaded forest conditions or sunny, open locations. Seedlings grow rapidly, reaching a mature height of between 75 and 100 feet within 70 years.
The aerodynamic and geometric properties of the single winged maple tree fruit inspired students at the University of Maryland to create the world's smallest controllable single winged rotorcraft. Its wing was designed to function in the same way a maple key samara performs a stable rotation as it descends to the ground. The device is a hand-launched robot which can hover and perform surveillance in defence and emergency situations.