Significant amounts of the symplast may be removed from a young tree without seriously disrupting the workings of the Second Law, since enough energy remains to keep the parts and processes of the tree system orderly. As trees grow and mature, however, the inner and oldest living cells begin to die, and nitrogenous substances move outward toward the cambial zone into the still-living cells. When cells die near the inner margins of the symplast, the dead material is called the protection wood. Heartwood is one type of protection wood. Sapwood is wood that has a symplast.
Protection is a static feature, whereas defense is a dynamic process.
The symplast in the sapwood maintains a defense potential, because it contains living cells. Protection wood has very little or no nitrogen or energy reserves. Materials called extractives might also impregnate the dead cells. Because the ratios are changing, the amount of symplast lost by removing living branches is also changing. As trees grow and age, the demands of the Second Law become more pronounced.