Oak tree growth is particularly rapid in youth but it gradually slows when the tree reaches 120 years old. Its smooth and silvery bark becomes rugged and deeply fissured with age and the tree may even shorten with age in order to extend its lifespan. Its fruit, the acorn,  is 2 - 2.5cm long and held tightly in a cupule, and as it ripens, the green acorn takes on a more autumnal browner colour. Eventually it loosens from the cupule and falls to the ground below. Most acorns never get the chance to germinate as they are a rich food source for many wild animals. Acorns need to germinate and root quickly to prevent drying out but following a successful germination, a new sapling will appear the following spring.