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.

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
Although we can think of cellulose as being the same chemically in both hardwoods and softwoods, a paper discussed at a conference a few years ago seems to indicate differently. You have to excuse the fact that I don’t remember the author and don’t have time to go searching. In this study, a fungi was given samples of pure cellulose from hardwoods and from softwoods. The fungi released different enzymes for each preparation (to degrade the cellulose) indicating that the cellulose was different. We just don’t know how it is different.

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
Phloem
(inner bark)
Pith
Cambium
Xylem
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.
Heartwood
Earlywood
Knot
Sapwood
Definitions in notes section
Latewood

10. Reaction Wood This is a very poor representation of a very bent tree
Tension Wood
(Hardwoods)
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.
Compression Wood
(Softwoods)
Tension or
Compression
Wood
Notes

11. 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
please check out the links on this page…also check out the pictures on each of these links.

12. Microscopic Structure
Microscopic structure of wood (Textbook of Wood Technology, Panshin, A. J., page 118
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.
Notes

13. 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

14. Microscopic Structure
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.
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
Notes

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