All oak trees produce acorns. The nuts vary in size and features but all are distinguishable. Leaves of different oak species vary greatly, but most are lobed. Two major groups of oaks are distinguished by the presence (red oak group) or absence (white oak group) of bristles on the tips of the lobes. Inconspicuous wind-pollinated flowers are produced in the early spring, before the trees leaf out. Oak wood is a major building and furniture timber.