Don from Ellisville, MO on 10/19/2013 5:38:31 PM:
Tennis-ball sized pods, if they were bright green, would be hedge trees, aka Osage orange or horse apples. The farmers would use their wood for fence posts because it is strong and rot resistant.
Smaller, dark green/brown pods would be walnuts in their husks. They are about the size of golf balls. Smaller yet are pecan and hickory, which do not pose much of a hazard but you will hear them popping as they shoot from under your tire. With the leaves falling, there are many hidden dangers so try to ride in the clear areas.