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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.Because the epidermis cannot do this, it has to be replaced by a periderm (see below).In lecture I have shown some slides of Vitis (grape vine) stem anatomy showing development of a cylinder of vascular cambium.Thus the secondary xylem cells produced in the fall of the year are much smaller in the radial dimension.Through the winter very few cell divisions occur because of the cold.And so while we are busy patting ourselves on the back or passing laws to prevent cloning, plants have been doing it naturally for millenia.
In looking at a cross section of a one- to three-year old stem, the secondary xylem and secondary phloem cells are easy to distinguish from their primary xylem and primary phloem counterparts.
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.
As the year progresses, the mineral content of the soil is depleted as the plant mines it out, and in late summer the water supply gets reduced through hot months.
This is followed by increasingly short days and the onset of winter cold.
In fact, of course, the pattern is not random, but it is clearly not in some radial alignment either.