Tracheids have obliquely-angled endwalls cut across by bars. The evolutionary trend in vessels is for shorter cells, with no bars on the endwalls. Image from Purves et al. Phloem cells are usually located outside the xylem. The two most common cells in the phloem are the companion cells and sieve cells .
Companion cells retain their nucleus and control the adjacent sieve cells.
Dissolved food, as sucrose, flows through the sieve cells. Phloem cells as seen in longitudinal section. Note the longitudinal view of the sieve plate inside the large sieve tube cell. Right image is a diagram of the longitudinal view of phloem cells. The epidermal tissue functions in prevention of water loss and acts as a barrier to fungi and other invaders.