1. Wood Chemistry PSE 406
Tree & Wood Structure

2. Agenda Tree components Macro wood structure Micro wood structure
Stem, crown, roots
Hardwood versus softwood
Macro wood structure
Reaction wood
Micro wood structure
Cell types
Cell wall layers

3. Why Wood Structure?
Chemical distribution is dependent upon macro and microscopic structure.
Tree species dependent
Dependent upon position in tree
Cell type dependent
Dependent upon position in the cell

4. Tree Structure I
It has been my experience that the majority of students taking wood chemistry can’t tell the difference between a hardwood and a softwood. In the next few slides I am going to present you with the layman's view of what is a tree.

5. Tree Structure II
In general, trees contain these structural components:
Stump/Roots
Stem (wood & bark)
Crown: live/dead branches, foliage (leaves/needles), flowers and fruits.
There are major chemical components which are found in all of these components. We will focus on the components found in the stem.
CrownThe crown, which consists of the leaves and branches at the top of a tree, plays an important role in filtering dust and other particles from the air. It also helps cool the air by providing shade and reduces the impact of raindrops on the soil below.
The leaves are the food factories of a tree. They contain chlorophyll, which facilitates photosynthesis and gives leaves their green color. Through a process called photosynthesis, leaves use the sun’s energy to convert carbon dioxide from the atmosphere and water from the soil into sugar and oxygen. The sugar, which is the tree’s food, is either used or stored in the branches, trunk and roots. The oxygen is released into the atmosphere.
RootsA tree’s roots absorb water and nutrients from the soil, store sugar and anchor the tree upright in the ground. All trees have lateral roots that branch into smaller and smaller roots and usually extend horizontally beyond the branch tips. Some trees have a tap root that reaches down as far as 15 feet. Each root is covered with thousands of root hairs that make it easier to soak up water and dissolved minerals from the soil. The majority of the root system is located in the upper 12 to 18 inches of soil because the oxygen that roots require to function properly is most abundant there.
Trunk/StemThe trunk, or stem, of a tree supports the crown and gives the tree its shape and strength. The trunk consists of four layers of tissue. These layers contain a network of tubes that runs between the roots and the leaves and acts as the central plumbing system for the tree. These tubes carry water and minerals up from the roots to the leaves, and they carry sugar down from the leaves to the branches, trunk and roots.
HeartwoodAs a tree grows, older xylem cells in the center of the tree become inactive and die, forming heartwood. Because it is filled with stored sugar, dyes and oils, the heartwood is usually darker than the sapwood. The main function of the heartwood is to support the tree.
Xylem/SapwoodThe xylem, or sapwood, comprises the youngest layers of wood. Its network of thick-walled cells brings water and nutrients up from the roots through tubes inside of the trunk to the leaves and other parts of the tree. As the tree grows, xylem cells in the central portion of the tree become inactive and die. These dead xylem cells form the tree’s heartwood.
CambiumThe cambium is a very thin layer of growing tissue that produces new cells that become either xylem, phloem or more cambium. Every growing season, a tree’s cambium adds a new layer of xylem to its trunk, producing a visible growth ring in most trees. The cambium is what makes the trunk, branches and roots grow larger in diameter.
Phloem/Inner BarkThe phloem or inner bark, which is found between the cambium and the outer bark, acts as a food supply line by carrying sap (sugar and nutrients dissolved in water) from the leaves to the rest of the tree.
BarkThe trunk, branches and twigs of the tree are covered with bark. The outer bark, which originates from phloem cells that have worn out, died and been shed outward, acts as a suit of armor against the world by protecting the tree from insects, disease, storms and extreme temperatures. In certain species, the outer bark also protects the tree from fire.

6. Tree Structure III Softwoods Hardwoods Trees containing needles
Typically retains needles over winter.
Pines, firs, cedars, spruce
Hardwoods
Trees containing leafs
Typically lose foliage in winter
Maple, alder, oak,

7. Tree Species Differences

8. Tree Composition
1. Values for branches, foliage, bark, and wood = % of tree above ground
2. Values for roots is a separate measurement = % of total tree

9. Macroscopic Structure
Annual Rings
Outer Bark
(dead, protection, high extractives)
Phloem
(inner bark)
(transportation of water
and nutrients)
Pith
Cambium
(growth, inward wood,
outward bark)
Xylem
=wood
Pith: This is the primary tissue in the center of the stem or root. This is the first years growth and is significantly different than the rest of the wood.
Cambium: This is a thin layer of growing tissue; this is where growth takes place in the stem. Cells growing inwards become wood and outward become bark. This layer is located towards the outside of the tree between the Xylem and the Phloem.
Xylem: Principle strengthening and water conducting tissue of stems, roots, and leaves. In layman's terms, this is the wood.
Phloem: This is the inner bark which contains living cells. Transportation of water and nutrients occurs in this area.
Outer Bark: This is the dead protective layer that you would recognize as bark. This layer is very high in extractives.
Sapwood: Outer (younger) portion of the woody tissue. Usually light in color; contains living cells. Functions in transport of fluids.
Heartwood: Inner (older) portion of the woody tissue. Usually dark in color; contains dead cells. Functions as supportive tissue.
Earlywood: Growth produced in the beginning of the growing season. Wide, thin walled cells for water transport. Lighter colored.
Latewood: Growth produced in the end of the growing season. Thin, thick walled cells for support. Darker colored.
Knots are portions of branches included in the wood of the stem or larger branch. Branches generally originate at or near the pith (central axis) of a stem, and the living portion will increase in size through the addition of annual woody layers which are a continuation of those of the stem. The included portion is irregularly conical in shape with the tip at the pith. The fibre direction is at right angles or oblique to the grain of the stem, thus producing local cross grain. A small knot may also be the result of a dormant bud.
Heartwood
(support, dead, dark)
Earlywood
Knot
Sapwood
(younger, light color, living cells, transportation)
Definitions in notes section
Latewood

10. Macroscopic Structure (2)
Heartwood
Sapwood
Earlywood
Latewood

11. Reaction Wood This is a very poor representation of a very bent tree
Tension Wood
(Hardwoods)
Compression Wood
(Softwoods)
We all know that trees like to grow straight. When some unnatural element like wind, snow, landslides, etc bend a tree, the tree will work towards straightening itself through addition of reaction wood. In conifers (softwoods), this material is applied to the bottom of the stem. As it is formed this tissue expands longitudinally thus causing the stem to bend upward. This type of tissue is known as compression wood. In hardwoods, the reaction wood is applied to the top of the top of the stem. As the tissue forms, its contracts thus pulling the stem vertically. This tissue is known as tension wood. Physically and chemically reaction wood varies significantly from normal wood.
Reaction wood is formed in response to mechanical stress, and helps to position newly formed parts of the plant in an optimal position. Reaction wood forms when a woody plant part is subjected to mechanical stress. This stress may be the result of wind exposure, excess of snow, soil movement, etc. There are two different types of reaction wood, which represent two different approaches to the same problem by these groups of plants:
In angiosperms reaction wood is called tension wood. Tension wood forms above the affected part of the plant, pulling it up. It is composed almost entirely of cellulose.
In conifers it is called compression wood. Compression wood forms below the bent part, pushing it up. Compression wood is rich in lignin.
As a rule, reaction wood is undesirable in any commercial application, primarily as its mechanical properties are different, also it responds differently to changes in moisture.
Tension or
Compression
Wood
Notes

12. Wood Microscopic Structure
Imagine that wood is made up of millions and millions of toilet paper rolls glued together. These rolls are the fibers that will make paper
Most often the ends of these tubes are sealed. There are small holes in the sides of the tubes to allow water to pass through

13. Microscopic Structure
Resin canals (epithelium parenchyma secretes resin epithelium parenchyma secretes resin)
Rays (transportation of water)
Tracheid (support, water transport, softwoods),
in hardwoods we have libriform fibers)
Pits (wholes, transport between fibers, different typs)
Tracheids: These are the supporting and water transporting cells found in softwoods. They are long and narrow cells.
Libriform fibers: These are hardwood cells which are somewhat similarly shaped as softwood tracheids. They function as supporting elements
Vessels: Short, wide, thin-walled cells found in hardwoods. These cells stack on top of one another and function as water conducting elements.
Pits: These are holes in the fibers which allow water to flow between fibers. There are different types of pits and different elements to the pits. For this class, the above definition is satisfactory
Rays: Ribbon shaped band of tissue which travels perpendicular to the annual rings. Tracheids, vessels and libriform fibers travel vertically in the tree. Rays travel horizontally for transport of fluids. Think of east-west and north-south freeway systems.
Parenchyma: These are short, thin walled cells used for the storage of carbohydrates. They are also living cells.
Resin Canals: These are tubular, intercellular space surrounded by epithelium parenchyma cells which secrete resin. When the tree is wounded, parenchyma cells release resin which travels down these canals to the wound.
Microscopic structure of wood (Textbook of Wood Technology, Panshin, A. J., page 118

14. Hardwood & Softwood Fibers
Softwood Cells
Source: Wood Chemistry, Fundamentals and Applications. Sjostrom page 7
Hardwood Cells
Source: Wood Chemistry, Fundamentals and Applications. Sjostrom page 10

15. Microscopic Structure
W-warty layer, thin,
storage of metabolites
S (S1+S2+S3)-secondary wall, the thickest,
microfibrils - opposite direction
P-primary wall, very thin, random microfibrils,
This drawing shows the different layers of the woody cell wall. Chemically and structurally these layers are greatly different.
ML: This is the middle lamella which is basically the space between cells. This region is mostly (70-80%) lignin. You can think of it as the glue layer between cells.
Primary Layer: This is the first layer laid down by the cell. It has a very thin wall. The squiggly lines you can see in the drawing represent cellulose microfibrils. In this layer, the microfibrils are randomly oriented which is necessary for this layer to expand during growth. This layer has a high content of lignin but not nearly has high as the ML.
Secondary Layer: This is the thickest layer of the cell. Even though the percentage of lignin is low in this layer, the thickness of the layer means that the majority of lignin in the cell wall is found here. In the drawing you can see three layers: S1, S2, and S3. In these layers, the micofibrils are going opposite directions like a sheet of plywood. This gives the cell wall strength.
Warty Layer: This is a thin amorphous layer located on the inner surface of the cell wall in all conifers and some hardwoods. It can be thought of as the garbage dump of metabolic chemicals
ML-space between cells, 70-80% lignin, glue
Structure of woody cell by Cote, This figure is used by almost every wood chemistry text. It can be found in Wood Chemistry, Fundamentals and Applications by Sjostrom on page 14.
Notes

16. Cell Cross Section Primary Secondary 1 Secondary 2 Warty Layer
Middle lamella