What type of cells primarily make up the xylem?

The xylem is a vital tissue in vascular plants primarily responsible for transporting water and dissolved minerals from the roots to the rest of the plant. The main cell types that make up the xylem are tracheids and vessel elements, which are specialized tube-like cells. These cells are designed to facilitate the efficient movement of water, creating a system that can transport large volumes under tension generated by transpiration.

In contrast, other cell types mentioned do not serve this primary function. Barrier cells are not typically associated with xylem structure and function; they may relate more to protective roles elsewhere in the plant. Photosynthetic cells, like mesophyll cells, are specialized for capturing sunlight and converting it into energy and are mainly found in the leaves. Storage cells are involved in the accumulation of nutrients and other compounds but do not play a direct role in the conduction of water and minerals, which is the key function of the xylem. Thus, tube-like cells are essential in defining the structure and function of xylem, making this choice the correct answer.