A tree during the winter months stands dormant, but during the spring (or
during the wet season, in some areas), it begins to grow. It actually
experiences a growth spurt—with new cells growing between the previous
year's tree ring and the tree's bark.
The cells that grow during this initial period are large. As the season
progresses, however, the cells that form are smaller and smaller. These smaller
cells appear darker.
The difference in appearance between the smaller latewood cells and the
following season's earlywood cells creates a striking delineation. The ring
represents one year in the life of the tree.