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Only 1 percent of a dormant mature tree is biologically living while the rest is composed of non-living, structural wood cells. In other words, very little of a tree's woody volume is composed of "living, metabolizing" tissue; rather, the major living and growing portions of a tree are leaves, buds, roots, and a thin film or skin of cells just under the bark called the cambium.
There are other living cells that are important for tree growth within the different parts of trees, especially in root tips, the apical meristem, and leaf and flower buds; however, these living cells make up a very small percentage of the total volume of a tree's cells. Instead, non-living or "dead" cells comprise most of the volume of a tree, providing vital structural support for the living cells.
Interestingly enough, trees start out in life as a germinating seed with every living cell in hyperdrive, but as a tree seed becomes a seedling, then a sapling, then a mature tree, its living contents become less and less as a percentage of the total volume. Trees increasingly lose their living cytoplasmic cells as metabolism ceases in each cell, and although they are no longer alive, these non-living cells now provide protection, transportation, and physical support for the living ones.
The Vital Role of Non-Living Cells
Without the support and structure provided by non-living cells, trees would likely die and certainly wouldn't grow quite as large as they do. This is because non-living cells provide a vital role in the process of how a tree grows - from the "heavy lifting" of holding up the tall branches to the tree's bark, which protects the thin layer of living cells underneath.
This supporting and protective wood is created by cambial-hardened cells produced on the inner and outer cambial layer and sandwiched between the outer cambial layer. As a result, the bark of a tree is a product of the ongoing process of creating sieve tubes to transport water and nutrients from the leaves to the roots and back.
The sound, non-living cells of a tree are very important to helping a tree stay protected, and the bark and structural cells serve as a line of defense against insects and disease that could affect the vulnerable living tissue of the cambium that maintains life throughout the tree.
New cells are formed and living cells cease metabolization as they transform into transport vessels and protective skin, creating a cycle of creation, rapid growth, slowing metabolism, and death as the tree climbs ever-higher into a healthy, full plant.
When Wood Is Considered Alive and Dead
For most intents and purposes, wood is considered to be the product of living cells in trees harnessing the environment around them to make proteins and form protective vessels and shells for the trees' sustained growth. Wood is only technically considered dead when it's separated from the tree itself, as it still serves a vital role in the plant's life when attached to living cells in the tree.
In other words, although wood is largely made of non-living cells - cells that no longer reproduce but instead transport nutrients to living cells - it is still considered "alive" if it is attached to the tree itself. However, if a branch falls off or a person cuts down a tree, the wood is considered "dead" because it no longer transports living matter through itself.
As a result, wood that has been separated from a tree will dry up as the protoplasm hardens and the protein turns into the wood one might use in a fireplace or for building a shelf. This wood is considered dead, though the piece it was once attached to - if still attached to the tree itself - is still considered alive.