The layered structure of a tree trunk is called "secondary growth". Trees increase in thickness over time through a process involving the cambium, which is a layer of actively dividing cells. This growth produces xylem, which transports water and nutrients from the roots, and phloem, which distributes sugars produced during photosynthesis. The different layers, including the bark, cambium, xylem (wood), and heartwood, serve various essential functions, including protection, nutrient storage, and support.