Vessels occur only in angiosperms. As lignin builds up in their cell walls, the contents die, leaving an empty space, the lumen. As the tissue develops, the end walls of the cells break down, leaving a long hollow tube through which water climbs straight up the plant. The lignin is laid down in a characteristic spiral pattern and, unlike cellulose of phloem walls, stains red so xylem is easy to identify in microscope sections.