Secondary Growth

Primary Plant Body: Stem

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. All of these tissues arose by cell division that occurred at the stem apex, and matured into the differentiated cell types and tissue layers from there.

Secondary Plant Body: Vascular

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. Additional xylem is needed. The root system will need to branch out and penetrate more soil to supply the water and mineral needs. 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. The stem will ultimately become a trunk. 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. 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. 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.

You might wonder how the cambium could expand across the pith ray which is already filled with mature parenchyma cells. The hormonal/developmental pathway is not completely clear yet, but those mature parenchyma cells dedifferentiate (go "backwards" in development) into meristematic cells. 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 other terms, plants are easily cloned. In fact asexual reproduction in plants is a well-evolved natural process. 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. People have been cloning plants from early human history onwards. I hope you will excuse a few chuckles from botanists as we watch zoologists and society react to what for us is really "old news."

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 (just one or two generations earlier) ice age from the land-bridge to North America and down past them through Central and into South America.

The annual growth layers can also be seen in secondary phloem...but of course the sequence is reversed. The youngest cells are toward the cambium and the older cells are toward the stem surface. So large spring phloem elements will appear to the inside of the previous year's small winter phloem elements. The annual growth rings in phloem are thinner because the cambium produces much less new phloem than it does xylem.

What is truly remarkable about the vascular cambium is that the initial cells can divide in different planes and the resulting derivatives have subsequent fates that are apparently determined by that plane of division. Here is a chart of fates for fusiform initials in the vascular cambium:

Plane of new wallDerivatives
Tangential2° xylem and/or 2° phloem
Radialfusiform initials
Transverseray initials

Furthermore, when the fusiform initial divides with a new tangential wall just one cell matures. How is it that the inner cell most frequently matures? How is it that the inner cell that matures always becomes xylem and never phloem? Similarly how is it that the outer cell that matures always becomes phloem and never xylem? Well, if you work out the answer, be sure to publish it. I think there is a Nobel prize waiting for that answer.

From the table above you also see a ray initial mentioned. What is a ray initial? Well this is another cell type found in the vascular cambium. This cell is nearly isodiametric in three dimensions. When it divides it produces derivatives to the inside and to the outside just like the fusiform initial. However, the derivatives follow a maturation pathway leading to parenchyma on both sides of the cambium. To the inside we have xylem ray parenchyma and to the outside we have phloem ray parenchyma. The derivatives from tangential ray initial divisions conduct water, minerals, sugars, and amino acids, etc. radially in the trunk of plants. The pith needs fuel for respiration, the cortex needs that and water and minerals from xylem too. So while conduction in xylem and phloem is mostly up-down, in a large diameter tree you also need radial transport to keep living cells in the stem alive. Ray parenchyma is responsible for this radial transport.

An interesting feature of the phloem ray parenchyma is that some of it divides before maturing to fill up the gaps in the phloem produced by the expansion of the secondary xylem just inside the phloem. These areas are called dilated phloem rays and their position can often show you the progress of expansion in the secondary phloem.

Secondary Plant Body: Dermal

Now, while all of this expansion is going on inside the stem, the dermal system has to cope with this expansion. We will look at Sambucus canadensis (elderberry) as our example. During the first year, this stem shows a developing cambium and secondary xylem and phloem, but the stem is still of close-to-herbaceous diameter... expansion is minimal. However, the epidermis is being stretched laterally as the expansion occurs. The layer of cortex just under the epidermis differentiates into a new layer: the periderm.

The periderm consists of a cork cambium and its derivatives. A layer of cortical cells becomes meristermatic (dedifferentiation again!), 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.

Maturing 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 another 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 (a botany 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.

The most-famous periderm, however, has to be the one on a tree known as Quercus suber. In fact its periderm gave the name to suberin from its specific epithet. The periderm on this tree gets very thick very fast, and can be harvested each decade by stripping it from the tree in sheets several centimeters thick. From this sheet one can punch out cylinders of cork and use those to seal the neck of wine bottles. The tree is meditterranean, so no surprise it grows in that region and in California. In class we spent 15 minutes or so discussing the etiquette of the cork in serving wine. I hope you took good notes and will never again make a fool of yourself in a restaurant.

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.

Primary Plant Body: Root

First we review the anatomy of a woody root in its first year of growth. This anatomy is similar to that of a herbaceous dicot. There is an epidermis, a cortex, and an endodermis. Inside that is a pericycle and primary xylem and primary phloem.

In woody species, a vascular cambium differentiates in the valleys between the primary xylem and primary phloem. It will expand laterally to form a continuous layer inside the root. The root vascular cambium operates in precisely the same manner as that in the stem, but it operates in a more temperature-controlled environment, and one in which water and mineral concentration changes are relatively minor. Thus the differentiation between spring and fall wood is far more subtle. Root wood has finer grain.

A periderm must also develop for rapidly-expanding woody roots. As the epidermal system in these older parts of the root are no longer absorptive, the development of the periderm also is virtually identical to that of the stem. The layer of the root where the cork cambium differentiates is the pericycle. As cork derivatives become suberized, the endodermis, cortex, and epidermis are cut-off from phloem supply and die.