Pine tree stem. Polarised light micrograph of section through a young (one-year) stem from a Pinus pine tree. At top right are parenchyma cells of the cortex, including resin canals (black-celled ovals). The next layer down is the vascular cylinder. This consists of an outer phloem (brown, yellow, and blue) above a thin purple ring of cambium. The inner xylem (the lower left half of the image) consists of tracheids (yellow) with medullary rays and more resin canals. At bottom left is the innermost tissue, the central pith.