Tracheids and vessel elements are associated with the xylem, which conducts water and minerals, while sieve-tube elements and companion cells belong to the phloem, which transports organic substances like sugars.
Tracheids, vessel elements, sieve-tube cells, and companion cells are components of two separate plant tissues involved in transport: xylem and phloem. Tracheids and vessel elements are part of the xylem tissue, which is responsible for the movement of water and minerals from roots to the rest of the plant. The xylem cells like tracheids and vessel elements are dead at maturity and form an extensive network for water conduction, while sieve-tube elements and companion cells are part of the phloem tissue, which transports organic compounds such as sugars and amino acids throughout the plant. Companion cells, which are alive at maturity, provide metabolic support for the sieve-tube cells, which lack some cellular organelles due to their specialization in transport.