Xylem is most often referred to as the water-transporting tissue in plants as it translocates water and inorganic nutrients (like nitrogen, phosphorous and potassium) from the roots to the leaves. The other component of vascular transport in plants is phloem whose main function is to translocate sugars, proteins and signalling molecules from source tissues (like the leaves) to sink tissues (like the roots). Xylem is composed of tracheids (long, thin cells) and vessel elements (short, wide cells) that share some key differences. Both of these cells are classified as dead cells that only act as pipes to transport water. Water moves between the tracheids only through pits, while between vessel elements water moves mostly through perforation plates (compound or simple) and subsidiary flow through pits. The xylem is supported by fibres and other lignified cells (especially the case for trees) to ensure stability. Vessel elements are the primary vessel type in angiosperms (broad-leafed plant species), which also contain tracheids.