Plants & Human Affairs
Cherries.wmf (7140 bytes) Plants & Human Affairs (BIOL106)  -  Stephen G. Saupe, Ph.D.; Biology Department, College of St. Benedict/St. John's University, Collegeville, MN 56321;;

Wood and Wood Products – A Quick Introduction

 I. What is wood?
    The simple answer - xylem cells. Recall that food and water are transported throughout a plant by the vascular system that is comprised of two major tissues, xylem and phloem. The phloem transports organic materials (i.e., food) from the sites of production (usually the leaves) to the sites of need (usually the roots). In contrast, xylem transports water (and dissolved minerals) from the roots to other parts of the plant. Wood is comprised of the cells of the xylem.

II. Xylemthe water transport tissue revisited
    There are four major types of cells in the xylem:

  1. Tracheids – cigar-shaped, tapered ends, lots of pits (thin circular areas);
  2. Vessel elements, - shorter, wider, ends attached;
  3. Fibers - long and skinny with thick secondary wall, mostly for support; and
  4. Parenchyma - alive, thin, store starch and other materials, lateral transport. The primary water transport cells are tracheids and vessels.

A few take-home-lessons:

  1. Gymnosperms (conifers like pines) only have tracheids for water transport whereas angiosperms (flowering plants like oaks, maples) have both and primarily rely on vessels for water transport.
  2. In the final stage of their development, vessels, tracheids, and fiber cells die. The cell contents are reabsorbed and a hollow, rigid cell wall remains.
  3. Because xylem cells have thick, rigid cell walls, an additional function of the xylem is support. That’s why redwood trees can get so big.
  4. The cell wall is very porous and allows water to freely pass through it.

III. Wood Structure
    In class we will look at a cross section of a woody stem and identify the following structures:

IV. Hardwood vs. softwood
    These terms refer to the species of tree from which the wood is obtained. Hardwoods are angiosperm (flowering) trees like the oaks, birches, maples, and basswood. Softwoods refer to conifers (gymnosperms) like pines, firs, and spruces. These terms are also roughly equivalent to the degree of "hardness" of the wood. In general, the hardwoods have harder wood than softwoods. However, there are hardwoods with "soft" wood such as basswood and the poplars, and similarly there are softwoods with relatively "hard" wood like southern yellow pine. The following table compares the two:

Comparison of hardwood and softwood
Feature Softwood Hardwood
species Conifers (gymnosperms) Flowering trees (angiosperms)
cell types Tracheids only Vessels & tracheids
texture Homogenous Heterogeneous
hardness "soft" easily split "hard"
uses Building, paper Furniture, fuels

V. Wood Appearance

A. Cuts – the "Jelly Roll" or Onion Model

  • Transverse (Cross) Section – across the stem, perpendicular to the long axis; annual growth rings appear as concentric circles
  • Radial Section – parallel to the long axis, through the center; grain pattern a series of parallel lines
  • Tangential Section – parallel to the long axis, anywhere but through the center; grain pattern wavy and variable, not all parallel

B. Boards

  • Quarter-sawn – radial cuts – note grain pattern (linear with perpendicular rays)
  • Plain-sawn – tangential cuts – not grain pattern ("wavy")

C.  Grain

  • due to annual rings and cell structure
  • runs in the direction of the tree
  • coarse grained wood usually with conspicuous annual rings, large pores

VI. Lessons from Wood

A. Forensics
    Wood structure has helped to solve several crimes including the conviction of Bruno Hauptmann for the abduction and murder of Charles Lindbergh’s infant son.  Lindberg's son was abducted on March 1, 1932 from his bedroom in their Hopewell (NJ) home.  By carefully studying the anatomical structure of the wood in the ladder left at the scene of the crime, technologist Arthur Koehler, was able to determine that:

B. Past History – Dendrochronology
    Trees rings provide a window on the past. Tree rings can be used to:

C. "Spirit of the Trees" (not on exam)
    This video features information about dendrochonology and the bristlecone pine, the oldest tree (organism) in the world (ca. 4000 years old). They are able to get so old because they have resinous wood resistant to decay, grow in arid region less prone to decay, and don’t drop their needles like regular pines.

VI. MN Forest Types  (not on exam)
    Roughly speaking, MN divided up into three main regions: prairie, deciduous forest, coniferous forest.

VII. Wood Products

A. Veneer – thin piece of board, glued onto a less expensive board; various methods of cutting, most commonly, the log is rotated after steaming and a knife essentially peels a thin layer off.

B. Plywood – glue odd numbers of veneers together

C. Particle/fiber board – small fragments glued together

D. Species/uses

E. Cork � bark of cork oak tree. Mediterranean. Subject to fire. Thick bark to protect. Many uses for the cork including stoppers (pressed directly from sheets of cork. The sheets are prepared by glue the ground up cork), insulation, shoes.

F. Bamboo – product from a plant in the grass family (not on exam)

G. Rayon/Cellophane/Acetates - derived from wood. Chips treated with sodium hydroxide. Then spun. (not on exam)

V. Paper Production (not on exam)

A. General
    International Paper, a local mill, makes coated papers like that used for Time, Newsweek, TV Guide and advertising supplements.

B. Process
    Debarked → chipped (takes about 8 seconds) → screened → TMP plant → refiners (plates with knives turn ground mix to pulp fibers about 1 mm long) → bleached to yellowish white (normally brown) → spread on machines → rollers → heat to remove water → reels → "super calendar"

C. Wood

D. Cool Trivia

E. Other Paper making
    International Paper uses a mechanical process. Other papers are produced by chemical means – sulfite (acids) and sulfates. These are used to separate the fibers to prepare pulp.

:   click here for exercises and study questions


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Last updated:  11/19/2008 / � Copyright  by SG Saupe /