What type of material forms the primary structure of xylem tissue?

The primary structure of xylem tissue is largely formed by lignin, which is a complex organic polymer that provides structural support and rigidity to the cell walls. In vascular plants, xylem is responsible for transporting water and nutrients from the roots to the rest of the plant. The presence of lignin in the cell walls of xylem cells allows them to withstand the negative pressure created during water transport and contributes to the overall strength and integrity of the plant's structure. This robustness is essential for the plant’s stability, particularly in tall and woody species, enabling them to grow and reach light without collapsing under their own weight.

While cellulose is another important component of cell walls and is abundant in many plant tissues, it primarily contributes to the structural integrity of non-woody plant parts like herbaceous stems and leaves rather than the specialized xylem tissue. Starch acts as an energy storage molecule and does not contribute to the structural composition of xylem. Proteins serve various functional roles in the plant but do not form the primary structural component of xylem tissue. Thus, lignin's role is crucial in defining the strength and functionality of xylem.