Obviously, as walls thicken, the space occupied by the living contents decreases and the ability of water and oxygen to reach the cytoplasm is diminished. It is literally an act of suicide that kills the protoplasm and ends wall thickening. Even so, the remaining hollow cell walls continue their supportive roles throughout the life of the plant. Most people are surprised to learn that, in a living tree, as much as 98 percent of its trunk and branches are composed of dead cells, including those that conduct water.
