The study of these layers in tree wood is called dendrochronology (dendro: tree, chrono: time, logy: study).A slide of an example slice from a tree from New Jersey is shown indicating seed germination in 1627, the marks of a range of fires is given, a lightning strike, lead balls from Revolutionary gunfire at the battle of Trenton are visible.
The secondary xylem cells that are produced in the spring and early summer of the year are large in diameter; water is abundant and minerals released from the snow melt make for optimal growth.This is sometimes called "wood grain." Each year a new layer of wood (secondary xylem) is produced.The thickness of this layer will vary year-by-year depending upon weather, competition, herbivory, and random events (fires, lightning strikes, pollution, floods etc.).As a woody plant grows, its size requires additional support and additional vascular pathways.The tree is developing a canopy of leaves that needs a vast amount of water and minerals from the roots. The root system will need to branch out and penetrate more soil to supply the water and mineral needs.The vascular bundles become aligned at the vascular cambium, and the cambium within the bundles (fasicular cambium) expands across the pith rays between the vascular bundles (interfasicular cambium).
This forms a continuous cylinder of cambium in the stem.
The ability of mature parenchyma cells to do this is truly remarkable.
In fact plant parenchyma cells have been called "totipotent" because they can indeed de-differentiate and redifferentiate to form an entire plant from just one cell. In fact asexual reproduction in plants is a well-evolved natural process.
Of course the larger root system is going to need additional carbohydrate to fuel its growth and active transport of minerals into the endodermis.
So additional phloem will be needed to move carbohydrate from the photosynthesis in the leaves to the root. As we shall see at the end of this lecture, the epidermis will need to expand to allow for the additional growth of secondary xylem and secondary phloem.
The secondary vascular tissues are aligned, forming radial files of cells that are produced from a single cambium initial cell that you can locate between the xylem and phloem.