The document discusses the parts of a tree trunk. It describes four main layers: the outer bark, which protects the tree; the cambium layer, which produces new cells and growth rings; the phloem or inner bark, which transports sap; and the xylem or sapwood, which transports water and nutrients up the tree. The xylem cells in the center eventually die to form the tree's heartwood. Overall, the document provides a concise overview of the key layers and tissues that make up a tree trunk and their functions.