Secondary cell walls are hardened with lignin and fuction as support. Transpiration-the evaporative loss of water from a plant. Explain how water is carried by the transpiration stream including an outline structure of xylem vessels, transpiration pull, cohesion and evaporation. See 16.2.5 for transpirational pull, cohesion, and evaporation. Xylem vessels-made of tracheids and vessel elements.
Cells are dead a functional maturity, but produce secondary cell walls before protoplast dies.
Tracheids are long thin cells. Have pits where water moves from cell to cell. Vessel elements are wider, shorter, thinner walled, have perforated walls to allow passage of water. Explain how the abiotic factors light, temperature, wind and humidity affect the rate of transpiration in a typical terrestrial mesophytic plant. That's your answer.