Final answer:
The xylem cells in a horsetail stem transport water and minerals from the roots to the stems and leaves, forming an essential part of the plant's vascular system.
Step-by-step explanation:
The tube-shaped cells of xylem in a horsetail stem, as seen in magnified cross-sectional views, serve a critical function in the plant. The xylem's main role is to transport water and dissolved minerals from the roots throughout the plant. This includes moving these vital substances up to the stems and leaves. The xylem tissue is composed of dead cells that have thick walls reinforced with lignin, which makes them effective at transporting water as their rigid structure can withstand the pressures involved. Though the cells are dead, they are essential to the plant's vascular system, which also includes phloem tissue responsible for transporting organic compounds.