There are 2 kinds of conducting cells in xylem tissue: tracheids and vessels. Both are relatively long, dead at maturity, and have thick, lignified secondary cell walls. Because water is pulled through at negative pressure (i.e., tension), these sculptured thick walls prevent cell collapse and also help support the plant.
Least specialised xylem, considered most early evolved, and are long, slender cells with tapered, overlapping ends. Water moves upward in roots and stems from tracheid to tracheid through thin areas in cell walls called pits.
Vessels are more advanced, occur in angiosperms and several other groups of plants, are shorter and wider with thinner walls, arranged end to end. Their end walls are partially or wholly dissolved, forming long, hollow vessels (like straws).