Text to the narration in the video: Plant ID: Woody stems and trunks.

Here is the text for my audio/narration to my YouTube video “Plant ID: Woody stems and Trunks”.  I wrote this myself; it is not cut and pasted from somewhere else, so if you can use this information to promote knowledge and spread love, then I grant you permission.  If you want to reference me or give me credit, that’s fine with me you can just mention/display credit to: “Crawlface” or you can provide a link to the crawlface YouTube channel, but if you do not or it is not that kind of relevant use or circumstance, then that’s ok too:

The outermost layer on the stems/trunk and roots of woody plants is called the bark.

The bark is traditionally divided into the outer and inner bark.  The outer bark is further described as having 3 distinct layers of tissue or cell types.

The outside or external part of the bark or phellem is the layer usually visible to us, which we can also think of as the surface.  It consists of non-living material or dead tissue also called cork.  The phellem is the outermost layer of the entire bark.

The middle layer of the outer bark is called the cork cambium or phellogen.  This tissue consists of undifferentiated cells that can divide and be used for various growth and maintenance or protection needs.  There are a couple of places or zones in the plant tissues where these kinds of undifferentiated cell divisions can found.  We call these kinds of tissues primary and secondary meristems, depending on the kind of growth they support.

The cork cambium is a secondary meristem or lateral meristem producing cork which adds to the plants lateral growth.  This is why you’ll hear cork described as a secondary tissue.

Cells that grow outward from the cork cambium are called cork and cells that grow inward form an interior tissue layer to the cork cambium called the phelloderm.

The phelloderm is the inner layer of the outer bark and has parenchyma cells that can be used by the plant for numerous needs and functions.  Think of them as unspecialized cells which may divide and become differentiated or adaptable to particular functions.

So basically, the outer bark can be divided into 3 distinct layers:

1-    Phellem, cork, dead non-living material, cells that have grown outward from the cork cambrium.

2-    Phellogen, cork cambrium, a meristematic zone, a layer of actively dividing undifferentiated cells that add lateral growth and protection by becoming cork cells.

3-    Phelloderm, cell layers that form interior to the cork cambrium, where you’ll find undifferentiated cells available to divide and assist in various functions.

Since the phellem is defined as the outer most layer of the bark, the phellogen and phelloderm can be defined as the middle of the bark.

The inner most layer of the bark is living tissue consisting of various living cell types such as the parenchyma cells I briefly mentioned.  This innermost layer of the bark is called the phloem.

The phloem is interconnected to the outer bark and functions as a distribution system carrying organic nutrients (mostly sugar) from photosynthesis at the leaves to other parts of the plant.  This transport of material is called translocation.

The phloem is arrangement in “vascular bundles” which are long continuous strands that extend through the roots and stems and reach into the leaves as veins of phloem tissue.

Vascular bundles contain both phloem and xylem tissues, as well as support and protecting tissues.  A main function of xylem tissue is to carry material from the roots up to various parts of the tree.

Generally the movement in phloem is multidirectional while the movement in xylem is only upward, and phloem is comprised of living cells while the xylem is mostly dead cells.

In the phloem, sieve-tube cells connect to form a tube that conducts material throughout the plant.  Mature sieve-tube cells lack certain specialized organelles, such as a nucleus and vacuoles, and rely on companion cells to help carry out cellular functions.

The vascular cambium is similar to the cork cambium and is also described as a lateral meristem, consisting of cells that are partially specialized or incompletely differentiated.  As these cells divide and specialize, they can either become ray cells or phloem and xylem cells.

Those cells that have not differentiated yet form a zone of cells on both sides of the cambium called the cambial zone.  This zone usually involves growth and replacement of the transport systems.  Growth inward toward the pith makes-up the secondary xylem and growth outward toward the bark makes-up the secondary phloem.  These meristematic cells increase the girth of the tree.

The vascular cambium also produces ray cells which help the lateral transport of materials.  Ray cells are oriented perpendicular to the stem axis and divide the stem into sections.  They can also be described as being perpendicular to the annual growth rings.  These radial extensions of cells are also known as medullary rays, pith rays, or wood rays.

The cambium produces volatile organic compounds that help protect the tree from pathogens.  Medullary rays are responsible for laterally transporting the protective compounds from the cambium to xylem vessels throughout the wood.

The rays also serve as storage centres for starch and lipids.

As the tree grows, the division of cells to the inside (inner-side) of the vascular cambium are called xylem and form the wood of the tree trunk.

The Xylem tissue made up of living cells that carries materials (mainly water) from the roots to the upper parts of the tree is called sapwood.

Sapwood also provides mechanical support and can store various materials for survival or maintenance such as converted sugar into starch.

Over time and as the tree continues to grow, the sapwood increases in organic compounds such as terpenes which help deal with pathogens and any injury or decay.  Eventually the xylem vessels of the sapwood become blocked and the living cells die.  This dead wood is called heartwood.

Because of the increased organic compounds, heartwood is highly resistant to decay.  Heartwood does not transport any water but it does provide mechanical support.

The pith is tissue (mostly parenchyma cells) at the centre of the stem, encircled by xylem, that can store and transport material throughout the tree.

In older tress the pith can dry out, die, or become replaced by the function of the xylem.

Vascular rays can be connected to the pith and provides a transport of materials.

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