All species of oak trees produce acorns. Acorns contain one seed, on rare occasion two seeds, encased in a shell. They range in size from 1 to 6 centimeters long and 0.8 to 4 centimeters wide. Depending on the exact species, acorns take up to 24 months to mature.
Acorns are not suitable for human consumption, but they act as an important food source for many animals big and small. These animals include birds and rodents, along with larger animals such as pigs, bears and deer. However, acorns can be toxic to horses and cows if consumed in large quantities.