A) The material that strengthens xylem vessels is lignin.

10. a) The material that strengthens xylem vessels is lignin.

b) The tissue that is removed when the bark of the dicotyledonous stem is ringed is phloem.

11. a) During the light stage of photosynthesis:
    • Chlorophyll absorbs light energy.
    • Water molecules are split (photolysis) into hydrogen ions, electrons, and oxygen gas.
    • Light energy is converted into chemical energy, forming ATP and NADPH.

b) The end products of the light stage are ATP, NADPH, and oxygen gas ($O_2$).

12. Two factors affecting enzymatic activities are:
    • Temperature
    • pH