Xylem are tubes that allows water and substances to go through a plant. They are made of cells that die through apoptosis (programmed cell death). This process thickens the side walls, remove the top and bottom ones, and progressively kills the cell, leaving a tube of all those cell walls in their place.
Source: Coursera's Understanding Plants - Part II: Fundamentals of Plant Biology Week 1 1.8 Vascular tissue - xylem
URL: https://www.coursera.org/learn/plant-biology/lecture/jZRR7/1-8-vascular-tissue-xylem