Readily visible are various layers of tissue, the most obvious being the wood portion with concentric annual rings. The principal function of the dead bark is to prevent the living tissues from drying out. Perhaps the least obvious but most important layer of tissue in the cross section is the cambium. It is a thin single cell layer of tissue between the inner bark and the wood, as shown in Figure 1. After cambial division each successive cell undergoes enlargement, wall thickening, and lignification.
Annual rings seen in the cross section are a chart of yearly growing activity.
In general, low density earlywood is produced from this time until about mid-July, when the cessation of leader growth and the maturation of new needles takes place. After mid-July new foliage ceases to be a net sink of photosynthate and becomes an exporter. Concurrently, there is a reduction in cambial division. Thus, more material becomes available for cell wall thickening, Also, reduced crown activity limits auxin production, which helps trigger the formation of high density latewood. Generally darker in appearance, latewood continues to be produced the end of the active growing season which, depending on the region, lasts about four months until late September, when the lower temperatures and a reduces photosperiod bring on tree dormancy.