Each year the tree forms new cells. These cells are arranged in concentric circles called annual rings or annual growth rings. These annual rings show the amount of wood produced during one growing season. In Canada and the North United States the growing season begins in the spring. At first the cambium produces numerous large cells with thin walls that form the springwood. If you look at a cross section of a tree this is the light-coloured ring. Towards the end of the summer the growth of the tree slows down. The cells manufactured at this time of year are small and have thick walls. They form the summerwood which appears as a darker ring on the tree cross section. One year of growth is therefore represented by a ring consisting of a light part and a dark part. Some people believe that the darker wood is not formed in winter. This is due to cambium being completely inactive in the winter. A new two-part ring is added the following year. The older rings are closest to the centre of the tree. The tree grows in diameter because it manufactures new cells around its circumference, not because the old cells get larger. The old annual rings form the heartwood of inactive cells. This is the dead part of the tree. The live portion includes only the most recent rings. The width of this portion depends on the age and species of the tree. This portion is generally one to seven centimetres in width. The dead wood is the largest part of the tree. It often takes on a darker colour. Annual rings generally exist in trees where the climate halts growth at some point during the year. Winter causes this shutdown in our country. For some countries it is caused by the dry season.