According to the law of conservation of matter, the number of atoms of each element involved in a chemical reaction stays the same no matter how they are arranged.
So, even though oxygen is combined with hydrogen on the reactant side, the total number of oxygen atoms remains the same after the reaction takes place. There are two atoms of oxygen on the reactant side, and two on the product side.
Xylem cells have no cytoplasm or end walls, meaning they form a tube through which water can pass freely to allow water transport. Lignin strengthens the cell walls, helping to support the plant.
