First we will review the layers of the primary dicot stem from outside to inside. On the exterior we have the epidermis which has developed from the protoderm. It has waterproofing and gas exchange functions. Beneath this is the cortex that may do photosynthesis in green stems but may also have respiration and storage functions in other species. There is a ring of vascular bundles. These consist of four layers: toward the epidermis a "bundle cap" of fibers, functional primary phloem, vascular cambium (aka cambium), and primary xylem toward the interior. In the center of the stem we have pith.
In lecture I have shown some slides of Vitis (grape vine) stem anatomy showing development of a cylinder of vascular cambium. The vascular bundles become aligned at the vascular cambium, and the cambium within the bundles (fasicular cambium) differentiates across the pith rays between the vascular bundles (interfasicular cambium). This forms a continuous cylinder of cambium in the stem.
The vascular cambium is meristematic; it is sometimes called a lateral meristem (not to be confused with lateral bud or lateral root). The cambium function is mitosis. The derivative cells of that mitosis form a layer on either side of the cambium. Derivative cells that mature toward the inside of the cambium initial cell become secondary xylem. Derivative cells that mature toward the outside of the cambium initial cell become secondary phloem. 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 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. The primary vascular tissues do not form neat radial files; the cells in the primary vascular areas appear to be much more "random" in pattern. In fact, of course, the pattern is not random, but it is clearly not in some radial alignment either.
Each year, the cambium produces a layer of secondary xylem and an layer of secondary phloem. Because more of the derivatives mature toward the inside, the layer of secondary xylem each year is much thicker than the layer of secondary phloem. Thus the layers of secondary phloem are much thinner as a group than are the corresponding layers of secondary xylem. Also, during the year, the secondary xylem cells show an orderly progression of radial cell thickness. 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. 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. 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. But with the snow melt of spring and releafing, the tree can once-again make very large cells.
The juxtaposition of the tiny fall-cells next to large spring-cells in the secondary xylem produces a visible layer in the wood. 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.). 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. Additional layers lead up to the year the tree died (gypsy moth infestation finished it off in the early 1980s). An additional slide shows some specimens of Bristlecone pine (Pinus aristata). These trees are the oldest single organism continuously alive that can be well documented. Individuals still alive today are over 4000 years old and we have dead individuals that go back to a time over 5000 years ago. Wood from the center of these trees was used to verify the calibrations of 14C dating. These trees were thousands of year old already when Jesus walked the earth. As their seeds were germinating humans were starting to record human history! Pyramids were built in Egypt. These trees of course were in the New World, so they could tell the story of the movement of humans after the recently-ended ice age from the land-bridge to North America and down past them through Central and into South America.
The periderm consists of a cork cambium and its derivatives. A layer of cortical cells becomes meristermatic, forming a cork cambium. This layer produces derivatives that mature to the inside, the phelloderm, and derivatives that mature to the outside, cork cells. Most of the derivatives mature to the outside, so many layers of cork cells are produced during a year. By comparison few if any cells mature to the inside, so the phelloderm is often just one or a very few cells in thickness. However, each cork cambium initial does produce these radial files of cell derivatives, so this periderm has the radial organization that compares to the secondary xylem, phloem and vascular cambium.
Divisions of the cork cambium initials tangentially and radially allow the periderm to keep up with later years of expansion in the trunk caused by the activity of the vascular cambium and its derivatives. The epidermis did not have this capacity. So the periderm is essentially a dynamic replacement for the rather static epidermis.
The cork cells become invested with suberin, a waterproofing substance, which provide that replacement epidermal function. Of course as the outer-most cork cells become suberized, the epidermal cells toward the outside are then cut-off from phloem supply of carbohydrates and die. Similarly, the cork cells themselves are waterproofed and therefore die at functional maturity; they are a relative of sclerenchyma cell types.
The gas exchange functions of the dead epidermis need to be replaced as well. The stem has cortex, phloem, and cambium cells (at least!) that need to carry out respiration requiring influx of oxygen and efflux of carbon dioxide. The cork cambium over-produces cork in certain areas of the stem surface, this causes ridges and cracks to appear. These features on the surface are called lenticels. The break in the cork layers permits gas exchange for tissues just under the periderm. The size and shape of lenticels is species-specific, and therefore probably under genetic control. Precisely how a cork cambium initial "knows" it is in the correct place to produce a ridge of extra cork relative to its neighbors, is one of the unanswered questions in plant developmental biology.
The periderm can become quite thick on plant stems. In Sequoia trees the periderm may be several feet thick. A slide showing Randy Moore (textbook author) in front of one of these trees, demonstrates the heat-insulating qualities of cork. This tree has survived forest fires and has become one of the largest individuals of this species. In fact one Sequoia tree, the General Sherman, is the largest single living organism on the planet. It is larger than 30 blue whales. These trees survived logging for two reasons. Humans lacked large-enough saws to cut through the tough and fibrous periderm. Also the wood of Sequoia warps badly if you are able to get it cut up. The lumber did not make it worth the effort to cut these down and saw them into boards.
Finally a slide is shown to help you understand the three parts of a young tree trunk. There is pith in the center. Surrounding this are annual layers of secondary xylem which we call wood. Finally the wood is surrounded by bark. Bark is the combination of dead epidermis, periderm, cortex, phloem (primary and secondary), and vascular cambium.
This page © Ross E. Koning 1994.
The MLA citation style for this page would be: Koning, Ross E. "Wood and Bark". Plant Physiology Website. 1994. http://koning.ecsu.ctstateu.edu/plants_human/secondary.html (your visit date).
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