Is it ensure, insure, or assure?
tuber tu·ber (tōō'bər, tyōō'-)
n. pl. tubers or tu·ber·a (-bər-ə)
A localized rounded projection or swelling; a knob, tuberosity, or eminence.
The thickened part of an underground stem of a plant, such as the potato, bearing buds from which new plant shoots arise. Compare bulb, corm, rhizome, runner.