Download The Biology and Usefulness of Bark

Eric Seibert
Biology 1035
11/18/2012
The Biology and Usefulness of Bark
Bark is often one of the things that people immediately associate with trees when it
comes to the appearance or biology of trees, and many people are unaware that the surface of
many stem-riddled plants is actually made of the same cork-derived material. Today, I would
like to delve into the complexities of wood bark and its biology, as well as the many crazy uses
humans have come up with for the material over the centuries. To understand bark, we must first
look at the core material that creates it, which is Cork.
Cork is the primary tissue which makes up bark tissue, starting as a single row of lateral
secondary meristematic cells, called the cork cambium, these meristems divide rapidly during
growing seasons, producing layers of periderm. The outer most layer of these meristems are
dead at maturity, and function as air filled protection for the cork cambium, primary phloem, and
everything else underneath. The different growth variations of cork can be observed across any
woody plants, and is apparent in the different coloring and textures of the surface of the different
plants.
When thinking about cork, the first thing that came to mind was commercialized cork,
you know, the kind used for stoppers in wine. This cork, which comes from the cork oak, is
composed of suberin, which is an impermeable material. For this reason, we have used it for the
purpose of stopping bottles, use in spacecraft heat shields, climate control and comfort in
footwear, and even lining for the clutch of mopeds to reduce friction. (1) It’s really a crazy thing
to think about all of the possibilities of this secondary meristem, found in plants all over the
world.
Eric Seibert
Biology 1035
11/18/2012
The magic maker in cork is still mostly a mystery, but now we are aware of its presence.
Suberin is the waxy substance on apples that creates the shiny effect on these fruits. The water
resistance of the substance is second to none and protects plants from insects and any other
outside pollutants it may come into contact with. In roots, suberin is inside the cell walls of the
endodermal cells and is known as a Casparian Strip or Casparian Band. Its purpose is to prevent
nutrients from entering the stele through the apoplast, and instead traverse the endodermis and
finally the symplast. This allows the plant to be selective with its nutrients intake. (2) In tree
bark, suberin is in the outer most layer of the periderm.
Now that I know how the outer most layer of bark is created, as well as its purpose,
dissecting the bigger picture can truly begin. What we know as bark consists of a number of
different tissues, most notably cork, followed by all its other interior tissues. Immediately
following the cork outer layer, we have the cork cambium, which is the end of readily visible
plant material. Following that, we sometimes have what is called the phelloderm, which is
another layer of cells created by the cork cambium, but is growing towards the inside, not the
outside. These three layers together make up what we know as the periderm. The periderm is
not only what we typically call bark, but in the root systems of some plants the skin is also
considered the periderm. A plants process of moving from having an epidermis to having a
periderm is simply in the age of that particular area of epidermis. Later in the year, stem
epidermis will be overtaken by layers of cork developed by the cork cambium. Within the
periderm are lenticels, which are important to the periderm because they act as pores to allow gas
exchanges to the outside. The shape of these lenticels is often used in characterizing and
identifying trees. These pores tend to form below a stoma or a group of stomata. The periderm
Eric Seibert
Biology 1035
11/18/2012
is a large component of bark, and protects the plant and its interior tissues very well indeed, but
what else is inside, taking advantage of all this protection?
The first layer after our beloved periderm is the cortex, which is actually eliminated
completely in some trees depending on the age of the growth being observed. This layer of cells
is snug in between the epidermis and endodermis, and is composed of mainly undifferentiated
cells. These cells are often large parenchyma cells, and often have thin walls. The cortex
transports materials through root diffusion and can contain starch, which is a common form of
food storage.
The next layer of cells is called the primary phloem, and is the primary source of sugar
conduction throughout the plant. The cells in particular that are responsible for this
transportation are called sieve tube members. These cells lack a nucleus and must be
accompanied by some type of companion cell to take care of most of its metabolic need. The
lack of a nucleus and other important organs is to ensure that these sieve tube members have the
least amount of resistance when conducting sugar-rich fluids. These cells are aligned in long
ranks of individual cells, arranged end to end for efficiency. The pores in these cells occur near
the top and bottom of the cell on what is known as a sieve plate, and act as the passage through
which the liquids pass from cell to cell.
Moving forward, our next layer of discussion is the vascular cambium. This layer of
cells is similar to the cork cambium in that it has a layer of cells growing inwards and outwards
to produce the specialized cells known as secondary vascular tissues. These tissues are the
secondary xylem and secondary phloem of the plant. The vascular cambium is a large part of the
primary meristem, and while maturing, grows secondary xylem inwards, and secondary phloem
Eric Seibert
Biology 1035
11/18/2012
outwards, which in turn separates the primary xylem and primary phloem. These cells originate
in the apical meristem on root and shoot tips.
The last layer of cells relevant to the growth of bark is the primary xylem. This is the
other transport tissue in plants, along with our first, phloem. Its primary function is the transport
of water throughout the plant, and is what we typically consider to be “wood.” The most notable
parts of the primary xylem are the tracheids and vessel elements, which are the transporting cells
of this tissue. The flow of water is regulated by the pressure on the roots, and when pressures get
higher, sap flows towards leaves and can often be forced out through what is called a hydathode.
(3) Over time, xylem dies off as the growing season’s progress and the tissue needing this flow
of water is pushed towards the outside. Over many years of this process repeating itself, trees
form what we like to call heartwood. (3) Heartwood has very little use to the tree, other than
adding support and strength to the living material of the plant. The living xylem is sometimes
referred to as sapwood, so as to easily distinguish it from its out of date, older, heartwood.
Now that we know the pieces, what are some of the uses besides what I’ve already
mentioned of the almighty bark? One that really caught my attention was the European Viking’s
use of young branches from a small leaved lime to produce cords and rope in the riggings of
many of their long ships. Other common uses that immediately come to mind are the use of bark
as mulch, which is needed for some plants which don’t grow in typical soil, and also the
innermost bark of pine is made into pine bread, or rye.
After learning of all the interior and exterior workings of bark, it is much easier to look at
trees in a new light. Not only do they have this outer layer of cork, but also a layer of cork
cambium, a potential phelloderm layer, a cortex, phloem, vascular cambium, and xylem layers.
Eric Seibert
Biology 1035
11/18/2012
These many parts that make up what I now know to be bark are all quite essential to the long life
of practically any plant. Conducting cells, a protective layer of suberin, and everything in
between really show that the outer most living layer of trees is really where all of the action
happens to keep trees alive. I learned a lot delving into the heart of what makes bark what it is,
and could find myself writing another paper on a more specialized type of bark in the future.
Eric Seibert
Biology 1035
11/18/2012
Bibliography
1. Gibson, Richard, Scorpex Wine Services (2005).
2. Kolattukudy, P. E. (1984). "Biochemistry and function of cutin and suberin".
Canadian Journal of Botany 62 (12): 2918~
3. Bidlack, James. Jansky, Shelley, Introductory Plant Biology 12th Ed. Ch. 6 Stems
(2011).