Diameter Expansion

How does a tree become larger in diameter? The answer is that a second layer of specialized reproducing cells, located just beneath the bark but outside the woody center, divide repeatedly to form new cells. This very thin (one cell thick) layer completely sheaths the main stem and branches and, at any point along its length, completely encircles the stem. This layer of reproducing cells is known as the cambium.

Cells of the one cell thick cambium have the capacity to divide repeatedly. They may divide in one of two ways. One type of division results in two new cells, one of which remains in the cambium with continued capacity for division and production of new cells; the other cell is added to either the wood or bark.

The second type of cell division in the cambium results in two new reproductive cells, both of which remain in the cambium. This type of division allows the cambium to increase in diameter as the tree diameter increases.

Once a new cell is created it immediately begins a process of development - a process that in total takes about three weeks to complete.

The new cell May divide again one or more times. Grows in diameter and /or length. Begins to thicken its cell wall as fibrils of cellulose and other compounds accumulate on the inside surface of the ultra-thin primary wall. Lignifies. Dies (loses nucleus and protoplast).

As the process is dynamic, with some steps occurring simultaneously, a look at a simplified representation of cambial cell division and development in the area just beneath the inner bark can aid in understanding: Consider the situation represented by line 1 to the right: A cambuim cell (C) lies between new wood (NW) cells to the left, and new inner bark or phloem cells (NB) to the right. Maturing and mature wood and bark cells lie to the left and right, respectively, of the newly-formed cells. The wood and bark cells, all of which were formed by division of the cambium cell, are in different stages of development. Those cells formed most recently (i.e. those closest to the cambium cell) have very thin walls and still have the capacity to divide again one or more times. Cells that have existed for a longer time (i.e. those furthest to the left and right) have grown in size and some have begun the process of cell wall thickening.

Consider now line 2. One of the new wood cells begins the process of diameter enlargement (E). The other new wood cell divides again to form yet another new wood cell. Both of these processes cause the cambium cell (C) to be pushed outward, expanding the diameter of the tree.

In line 3, we see that one of the wood cells that was previously expanding is now developing a thickened secondary wall. Another of the expanding cells has continued to expand. In addition, one of the newest wood cells has begun to enlarge, while the other has divided again. At the same time the cambium cell (C) has divided, forming two cells, one of which becomes a new wood (NW) cell with the other remaining a cambium cell (C). Again the tree diameter expands. The process as outlined above goes on and on during the growing season, with the cambium producing new wood and new bark cells.

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