Cross-section through the trunk of a hardwood, Quercus robur, the pedunculate oak. The difference between the heart wood (dark) and the sap wood (light) is seen clearly. Sap wood is saturated with water and contains the living cells of the conducting tissue. After 10-15 years sap wood is converted into heart wood, which is a completely lifeless, dry tissue. The concentric rings here correspond to annual growth rings, a count of which would indicate the age of the tree. The cracks visible in the trunk are due to drying.