[B] Companion cells
Tracheids and vessels are the xylem elements and are related to the transport of water. They are long tube-like structures with partially or completely breached walls to form water pipes (in vehicles) and pits in the cell wall (in tracheids) to conduct water. Sieve tubes are thin tube-like structures whose end walls are perforated with many holes and are called sieve plates. They are phloem elements and are the main food carrier elements. Associate cells contain many mitochondria and ribosomes and are the supporting units of sieve tubes.