Morphology of Tree Rings

This cross section of a tree trunk shows the difference in the types of "wood".

  1. Heartwood - xylem that has filled with organic material, plugging it up and making it appear dark.
  2. Xylem - active xylem. While these cells are not alive, they are open tubes for water transportation.
  3. Cambium- the thin layer that produces growth in diameter by forming new cells.
  4. Phloem - the active phloem serves for food transportation.
  5. Bark - the outer edge of the phloem produces cork, dead cells that protect the living tissue beneath from water loss. As the tree increases in diameter, the cork splits and falls off of the tree, to be replaced from cells beneath.
Each year a tree adds a layer of wood to its trunk and branches thus creating the annual rings we see when viewing a cross section. New wood grows from the cambium layer between the old wood and the bark. In the spring, when moisture is plentiful, the tree devotes its energy to producing new growth cells. These first new cells are large, but as the summer progresses their size decreases until, in the fall, growth stops and cells die, with no new growth appearing until the next spring. The contrast between these smaller old cells and next year's larger new cells is enough to establish a ring, thus making ring counting possible.
In this drawing of magnified cells, the difference in cell size is clearly visible.

Growth is to the right in the diagram.

Three full years of growth are shown.

Dendrochronology is the study of climate history using tree rings.