Phloem is one of two main parts of the transport system in the roots and stems of plants, the other being the xylem. It consists of vascular tissue that conducts synthesized foods, including sugars, proteins, and some mineral ions, through the plant. It is characterized by the presence of sieve-tubes and, in some plants, companion cells, fibers, and parenchyma cells. Phloem is of two kinds: primary, formed by differentiation from procambium, and secondary, additional phloem produced by activity of cambium.