Cut a tree trunk across and you see rings of different colour and texture. Those rings are layers with different jobs. From the outside in:

• Outer bark. Dead cells, often corky, that protect the tree from fire, insects, drying out and knocks. A tree sheds or splits the outer bark as the trunk gets wider.
• Phloem (the inner bark). A thin layer of living tubes that carries sugar made in the leaves to the places that need it: roots, buds, flowers and fruit. Sugar travels from where it is made (a source) to where it is used or stored (a sink), so it can move down or up.
• Vascular cambium. A layer only a few cells thick, between the phloem and the wood. Its cells keep dividing. Some of the new cells become wood on the inside; others become phloem on the outside.
• Xylem (sapwood). Tubes and fibres that carry water and dissolved minerals upward from roots to leaves. Most of what we call wood is xylem. Its water-carrying cells are dead and hollow when they work, like pipes.
• Heartwood. Older xylem near the centre that no longer carries water. It is often darker, filled with resins and tannins, and it gives the trunk strength and resistance to rot.