They have thick, lignified walls that allows them to conduct water vertically , and provide structural support . The alternation of favorable and unfavorable environmental conditions causes changes in wall thickness and cell diameter. Narrow, thick-walled cells are produced during stressful periods like winter or dry seasons. Tracheids have large pits in their cell walls which are used to conduct water horizontally between cells and different regions of the plant. The pits are more concentrated on the end walls where tracheids overlap .
Pits also occur on lateral walls but are less concentrated.
Tracheids are dead at maturity and their contents are governed by the laws of Physics. Rays are composed of living Parenchyma cells. Rays are one cell wide and 5-10 cells high. They connect the living Phloem with the dead Xylem . The rays facilitate radial conduction of water from the xylem to the phloem and may contain a few tracheids.