Lime tree stem. Polarised light micrograph of a cross-section through the stem of a lime tree (Tilia cordata), showing its internal structure. In the centre of the stem are large, thin-walled cells, which make up the pith (blue). Outside the pith is a single ring of wood (pink), which means the stem is one year old. The thin black circle outside the wood is the cambium, which produces a layer of wood each year. The radiating rays outside of the wood are the phloem, which conducts food around the tree. The phloem also contains layers of fibres to support the young stem. The outer bark consists of cork and the remains of the cortex.