Phloem is known as a complex tissue made up of sieve tubes (transporters), companion cells (metabolic regulator), phloem fibers (mechanical strength), and phloem parenchyma (compound storage). Sieve tubes are a series of cells that are joined end-to-end. It contains a thin cell wall, and its lumen has a sap-like substance that is comprised of proteins. Companion cells are specialized parenchyma cells that are elongated, thin walled and have a distinct nucleus. Phloem fibers have thick walls, and are also elongated cells. These cells can be found in the stem. Phloem parenchyma are living cells with thin walls, and they create packing tissue between all different types of cells in the plant.