The common oak is a deciduous broad-leaved tree in the Fagaceae family. The crown is large and branched; when looking from a distance, it almost looks like a sphere. The bark is greyish-brown, thick, and with deep longitudinal fissures (covered with a rough bark). The bark of young trees is grey and smooth. Oaks are unicameral trees. A freely growing oak reaches the age of bloom at 20–25 years; in a forest, however, as late as at 50–60 years. The common oak is an anemophile. Oaks play an important role in ecosystems. Numerous insects live in their leaves, flowers, and acorns. Acorns are a valuable source of food for small mammals and birds, who collect them for the winter.