Up Wood Cells Softwood Cells Hardwood Cells Abnormal Wood Bark

 

Classification

A. Hardwoods - deciduous or broadleaf

B. Softwoods - evergreen or conifer

These names are misnomers: there are exceptions to each of the above names.

Jerusalem thorn  -- narrow leaves

yew -- fleshy body with seed inside

live oak -- always green

larch -- drops needles in fall

Pacific yew -- harder than oak

balsa -- softest wood in the world.

Can be separated botanically:

A. Hardwoods - angiosperms ( seed in fruit)

B. Softwood - gymnosperms ( naked seed)

See Table 1.1 on page 4 of Textbook.

Depending on location which is predominate and/or economically important. See Figure 1.1. on page 6 of Textbook.

********************************************************************************************

                                     

When looking at the end of a log, one sees a series of concentric rings. Usually there is a lighter portion and a darker portion. Normally the two portions together make up one annual growth ring. See the textbook for the exceptions.

 

Earlywood vs Latewood

Springwood vs Summerwood

Two sets of terms meaning the same thing. Earlywood and springwood are same and latewood and summerwood are the same.

A. Softwoods -- same cell type (trachied) but different cell size -- a good growing season results in an increase in the percent earlywood


B. ring-porous Hardwoods -- large thin wall cells (vessels) transitioning to smaller vessels and mainly fibers -- a good growing season results in the same amount of earlywood vessels but an increase in the amount of latewood fibers.


C. Diffuse-porous Hardwoods -- medium to small vessels throughout year -- though latewood is mainly fibers.

                               Generally speaking the cells are as such:

***********************************************************************************************

 

Orthotropic

Three mutually perpendicular axes of symmetry -- properties (strength, shrinkage, etc.) of wood are different along each axis.

 

L = Longitudinal -- look at transverse or cross section -- along the grain from roots to top.

R = Radial -- from center out to the bark -- perpendicular to growth rings.

T = Tangential -- tangent to growth ring -- around the tree.

*************************************************************************************

How do the axes look at the cellular level?

 

Softwood Block                                                                Hardwood Block

Images taken from Panshin and DeZeeuw -- Textbook of Wood Technology -- McGraw-Hill.

 

 

Tree Growth

Meristematic Regions -- regions in which cells divide repeatedly.

 

Apical Meristem -- growth region at the apex or tip of main stem.

As cells divide, the apical meristem moves up providing height growth.


If you attach a wire clothes line to a tree, six feet above the ground, and come back 40 years later, the wire will still be only six feet above the ground.

 

Lateral Meristem called Vascular Cambium -- provides horizontal growth.

The cambium can divide in two ways.

PERICLINAL -- increase in diameter -- one new cell remains cambium and the other new cell becomes new tissue.

If new tissue cell is on the outer side of the divide, it becomes phloem or bark. I f new tissue cell is on the inner side of divide, it becomes xylem or wood.

ANTICLINAL -- increase in circumference -- tangential growth -- both new cells become cambium and are side by side.

SEE Figure 1.9 on Page 15 and Figure 1.10 on page 16 of textbook.

***********************************************************************************

SAPWOOD -- the living outer portion of the tree.


HEARTWOOD -- dead inner portion of the tree.

Heartwood no longer functions physiologically but does provide mechanical support to the tree.

After a period of time, the cells in the center of the tree die. When this happens, extraneous materials are left in the cell cavities which may cause a change in properties:

A. darker color but not necessarily

B. decay and/or insect resistant but not necessarily (See Table A.4. on Page 514 of Textbook)

C. usually penetration is difficult but not necessarily

D. slow drying

E. distinct odor but not necessarily

F. higher dry weight per volume.

*************************************************************************************

RAYS -- tissue with the cells going horizontally from the center of the tree to the bark.

These are present in all woods and are used for transferring sap from the cambium to the center of the tree.

Some rays may be only a single cell wide and 6 to 8 cells high; while others may be 6 to 8 cells wide and 20 to 30 cells high. Depends on the species of wood.

When wood dries, stresses develop that may exceed the strength of the rays causing splits to develop along the ray.

*************************************************************************************

Most trees produce wood that the long axis of the cells grow in the longitudinal direction of the tree bole. This is called straight grain. Some trees produce grain that goes at an angle around the bole. This is called spiral grain. Still other trees produce spiral grain in one direction for a few years and then reverse directions for a few years and then reverse again. This is called reversing spiral grain or interlocking grain.

SEE Figures 2.12, 2.13, and 2.14 on Pages 41-42 of the Textbook.

*************************************************************************************

BRANCHES -- start at the pith or center of the stem. As the tree continues to grow in diameter, the limb continues to larger also. As long as the limb remains alive, it will be embedded in the wood. If the limb is pruned (cut-off) or it dies and breaks off, the tree produce wood that will cover the limb stub and start to produce clear wood over it.

SEE Figures 2.15 and 2.16 on Pages 43 of the Textbook.

**************************************************************************************

Chemical Composition of Wood

Tables 3.1 & 3.2 on Page 48 of Textbook

Element

Percent

Carbon

49
Hydrogen 6
Oxygen 44
Nitrogen slight
Ash 0.1

 

Organic Compounds Hardwoods Softwoods
Cellulose 40-44% 40-44 %
Hemicellulose 15-35% 20-32%
Lignin 18-25% 25-35%

 

Cell walls are made up of layers which are made up of microfibrils

Microfibrils are clusters of cellulose chains. Microfibrils have two regions: crystalline region where the cellulose chains are fairly straight and organized and surrounded by hemicellulose; amorphous region where the cellulose chains go in various directions such as spaghetti. Lignin, a nonstructural encrusting substance surrounds the microfibrils and fills the amorphous region. 

 

**************************************************************************************

Study and know Figure 3.8 on Page 57 of Textbook

Primary Wall -- pectin-rich membrane

Secondary Wall -- 3 layers

        S1 -- Microfibrils at 50-70 degrees  --  4 to 6 layers

        S2 -- Microfibrils at 10 to 30 degrees in opposite direction -- 30-40 layers in earlywood and 150+ layers in latewood

        S3 -- Microfibrils at 60 to 90 degrees in original direction -- 4 to 6 layers

Compound Middle Lamella -- material joining cells together

Lumen -- cell cavity

**************************************************************************************

Pits -- openings on radial face

In the radial face of cells there are gaps in the secondary wall that allow moisture movement from one cell to another. Parenchyma cells (rays) have simple openings and conductive cells like tracheids, and vessels, have bordered openings.

When two cells are in contact the openings forms a pit.

Study and know Figure 3.14 on Page 61 of the Textbook.

Remember that the drawing of each type of pit is made up of the cell walls of two cells joined together.

 

 

 

Up Wood Cells Softwood Cells Hardwood Cells Abnormal Wood Bark