My wife and I found this beautiful pine tree that had been cut down and into pieces for easy disposal. This photo was taken at The F.R. Newman Arboretum on the University of Cornell campus, Ithaca, NY. It's a wonderful place to visit: http://www.cornellbotanicgardens.org
This is the cross-section that we are going to look at in some detail.
1) Bark (dead phloem).
2) Phloem (small dark ring directly below the bark). Phloem is composed of tubes that carry nutrients from the leaves DOWN to the ground. Phloem tubes eventually die and turn into bark.
2A) Directly below the Phloem is a thin layer of Cambium (it's hard to see in this photo). The Cambium produces cells, the outer cells turn into Phloem and the inner cells turn in Xylemm.
3) Xylem (all the rest of the wood down to the center of the tree). Xylem is composed of tubes that carry water and nutrients UP from the ground to the leaves.
4) Sapwood - xylem that is still alive.
5) Heartwood - xylem tubes that have died.
6) Pith - the first year of the trees growth.
7) Branches (2 of them) - Once a branch starts to grow the part of the branch that connects to the tree is eventually surrounded by new growth. In a piece of lumber where you see "knots." the knots are pieces of a branch.
Xylem tubes, formed from the cambium, grow in rings. Some rings are darker in color than others. The dark colored rings grow in the spring (when the temperatures are cooler, the tubes are smaller and closer together, thus they look darker). The lighter colored xylem rings grown in the summer when the temperatures are warmer and growing temperatures are generally better. One dark ring plus one light ring equals one year's growth. Some rings are larger than others, those indicate better growing conditions.
Closeup of an enclosed branch:
1) Xylem rings of the branch.
2) Bark, phloem and cambium of the developing branch.
3) Xylem rings that, year by year, eventually grow around the branch.