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 W.H. Freeman and Sinauer Associates, used by permission. 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. 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. Right image from W.H. Freeman and Sinauer Associates, used by permission. The epidermal tissue functions in prevention of water loss and acts as a barrier to fungi and other invaders.