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.