No, xylem fibres do not have a nucleus.
Xylem fibres are a type of sclerenchyma cell found in the xylem tissue of plants. Their primary function is to provide structural support to the plant. Unlike xylem parenchyma cells, xylem fibres are dead at maturity. This means they lack protoplasm and, importantly, a nucleus. The absence of these cellular components allows for efficient water transport through the xylem vessels, which are the main conductive cells of the xylem. Xylem fibres provide mechanical strength to the xylem tissue, which withstands the negative pressures created by the transpiration pull.
In contrast, other plant cells such as cambium cells, root hair cells, and companion cells are living and contain a nucleus, which is essential for their metabolic functions and cell maintenance. The specialized structure and composition of xylem fibres are optimized for their role in support and do not require the metabolic activity associated with a nucleus.
