Cross-section illustration of part of a tree trunk. Under the bark, there is a light-coloured layer where the descending sap circulates. The concentric layers underneath are the rings, the lighter part of each ring is springtime wood, the darker is summer wood. The outermost rings are sap wood in which rising sap circulates. The innermost rings are the hardest part of the wood : the heart wood. All the layers of the wood are crossed through by medullary rays that allow the cohesion of the trunk (horizontal lines from the centre of the trunk). The rings are threaded through with vascular rays (vertical lines).