What is the name of the outside layer of a tree trunk or stump where sap flows?

The outside layer of a tree trunk or stump where sap flows is known as the cambium layer. This layer is a thin zone of actively dividing cells that is responsible for producing new phloem (the tissue that transports nutrients and sap) and xylem (the tissue that transports water). It plays a vital role in the growth of the tree, allowing it to increase in diameter over time.

While the phloem layer is important for nutrient transport, it lies just outside of the cambium. The cambium itself serves as the growth layer situated between the bark (which is the outer protective layer of the tree) and the wood (the inner supportive and transporting structure). Therefore, it is the cambium that is actively involved in the process of sap flow and the production of new layers as the tree grows. This makes it a crucial component in the overall health and vitality of the tree.