Exam #5 Hints

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Hints For Botany 115 Exam #5

Wood Anatomy and Plant Fibers

For answers to many of the questions please refer to the Reading List for Exam #5.
Also try the Wayne's Word Index & Economic Plant Families. Answers to most of
the diagrams are in the Wood Anatomy and Cell Structure Of Stems. Many of the
answers can also be found in Plant Fibers Used For Paper, Cordage and Textiles.
Also try your luck using the interactive Wood and Plant Fibers Crossword Puzzle.

Questions 1-77: See illustrations in Wood Anatomy and Cell Structure Of Stems. You might want to download and print out the PDF version of Exam #5 because the illustrations are much higher resolution. To calculate the age of a stem, see the following illustrations:

Questions 40-50: Cross section of a young, woody dicot stem (basswood):

To calculate the age of this young stem cross section, just count the number of thick purple rings (bands) outside of the yellow pith. Only count the layers of woody growth (xylem tissue), do not count the central core of pith. On older stems where the central pith region has been replaced by xylem tissue, you must count the central core of wood as the first year of growth. See the following photo image:

This basswood (Tilia americana) trunk cross section has 24 distinct annual rings. The central core of wood (#1 in close-up photo) counts as the first year of growth since the pith is no longer present. The smaller series of concentric rings (knot) at the bottom of the photo is a lateral branch embedded in the main trunk.

See How Trees Are Age-Dated Using An Increment Borer
Age-Dating Ancient Wood Using The Cross-Dating Method

Questions 51-68: Cross section of a ring-porous, woody dicot stem (basswood):

It is more difficult to determine the years of growth in this stem cross section, so I have numbered the annual (growth) rings. Each year of growth starts out with larger spring cells (vessel elements) and ends with smaller (more dense) summer cells (mostly tracheids).

Questions 69-84: Cross section of a gymnosperm stem (pine):

In this illustration the smaller (more dense) summer cells during the last year of growth are not shown; therefore, this stem is in the spring of its last year of growth. Starting with the spring cells just outside the pith, the xylem tissue clearly shows four springs and three summers; therefore, this stem is in its fourth year of growth.

See Resin Ducts In The Phloem Of A Poison Oak Stem

Questions 85-126: See illustrations in Wood Anatomy and Plant Fibers Used For Paper, Cordage & Textiles.

Questions 85-97: Three dimensional illustration of a block of oak wood:

The transverse (x.s.) plane of this block of oak wood shows nine years of growth (9 annual rings). The central cylinder of growth (red number 1) is counted as the first year of growth. Unlike the young stem cross sections above, this is not the pith. The pith is reduced to a mere central dot in the illustration; therefore the central cylinder of woody tissue (red number 1) counts as the first annual ring. The succeeding years of growth (annual rings) are shown by the red numbers 2-9.

A block of oak wood showing the tangential plane (T) and the radial plane (R). The parallel lines on the radial side are annual rings. The blotches of cells at right angles to the annual rings are rays (ribbonlike aggregations of cells extending radially through the xylem tissue).

Cross section of a pair of oak bookends showing the prominent rays. Each ray (blue X in photo) starts in the center and extends radially like the spokes of a wheel. Rays are composed of bands of thin-walled parenchyma cells that conduct nutrients and water laterally in a stem. Because their walls are not heavily lignified like the surrounding xylem cells, ray cells disintegrate in dead wood and often result in radial splits in the wood. One notable comment about these bookends is that they are made of petrified oak wood. Millions of years ago, the original cells in this trunk were completely replaced by minerals. This piece of oak has literally turned into stone.

Questions 98-103: Read about specific gravity in the Hardwoods Article. Specific gravity questions about an unknown block of wood:

The hardness and weight (mass) of wood is determined primarily by the lignin content of the cell walls. Lignin is a complex phenolic polymer composed of benzene rings. It is not a polysaccharide. Probably the best way to appreciate the relative hardness of different woods is the concept of "specific gravity," a numerical scale based on 1.0 for pure water. Without getting too mathematical, the specific gravity of a substance can easily be calculated by dividing its density (in grams per cubic centimeter) by the density of pure water (one gram per cubic centimeter). Since the units of measurement cancel out, and since the density of pure water is 1.0, the specific gravity of this unknown block of wood will have the same numerical value as its density. To find the density of the block in grams per cubic centimeter, divide its weight in grams (76.5) by its volume in cubic centimeters (85). The value for the specific gravity is the same as the density, except there are no units of measurement. [I.e. the units of measurement are cancelled out by dividing the density of the block by the density of pure water.]

Questions 104-110: See illustration of logs cut at the saw mill in Wood Anatomy:

Radial sections are made along the rays or radius of the log, at right angles to the annual rings (see Figure B). [Note that the radial cuts are mostly at right angles to the red annual rings in Figure B.] This plane is also called quarter-sawed lumber because the logs are actually cut into quarters. The rings appear like closely-spaced, parallel bands (See Photo 1 Below). Since relatively few, large, perfect, quarter-sawed boards can be cut from a log, they are more expensive. Because the dense, dark summer bands (annual rings) are closely spaced, this plane is also more wear-resistant. Note: Tangential sections are made perpendicular to the rays and tangential to the annual rings and face of the log (see Figure A). This plane is also called slab-cut or plane-sawed lumber. The annual rings appear in irregular, wavy patterns (See Photo 2 Below). This is the plane in which most plywood and lumber is cut at the saw mill.

Photo 1. Radial plane of ponderosa pine (Pinus ponderosa) showing the closely-spaced, parallel annual rings. This is also called a quarter-sawed board. It is more resistant to wear because the dense (dark) summer bands are very close together.

Photo 2. Tangential plane of Douglas fir (Pseudotsuga menziesii) plywood showing the attractive, wavy pattern of the annual rings.

Petrified sequoia wood (Sequoia) showing perfectly preserved tangential (T) and radial (R) planes. This 15 million-year-old petrified wood was uncovered from its ancient tomb of flood sediments and lava flows near the Columbia River Gorge in central Washington. Sequoia trees once grew wild in this region 150,000 centuries ago.

The narrow bands of summer wood in this old piece of charred lumber are more resistant to wear compared with the wider bands of spring wood. The summer wood is harder because the cells are smaller and more dense.

Questions 111-115: Refer to hints for questions 98-103 above. Also refer to Wayne's Word article about Hardwoods and the Cell Structure Of Stems.

Questions 116-125: For questions about plant fibers please refer to the Wayne's Word article about Plant Fibers Used For Paper, Cordage & Textiles. Also refer to the Wayne's Word article about the Cellular Structure Of Stems.


Xylem Tracheid	Xylem Vessel Element

A tracheid and a vessel element, two types of water-conducting cells found in xylem tissue. Long chains of vessel elements connected end-to-end are called vessels. Pits in the walls allow water molecules to pass laterally through adjacent xylem cells, as a steady chain of water molecules moves upward through the xylem (vascular) tissue. Generally, most cone-bearing gymnosperm trees do not have vessels. Instead, their xylem tissue is composed primarily of tracheids. Gymnosperm wood is generally considered to be a close-grained softwood. Both gymnosperm and angiosperm woods are converted into pulp for making paper products.

Cross (transverse) section of oak wood (Quercus agrifolia). The annual rings appear like concentric bands and can be counted to age-date the tree. This is a ring-porous wood, with bands of large, porous spring vessels. Smaller, dense tracheids and vessels occupy the wider gaps between the spring bands. In this wood, the spring vessels actually appear darker and are easier to count. In pine wood, the darker, summer bands are easier to count.

This small block of angiosperm wood is used for an aquarium aerator. Fine jets of air bubbles come out of the porous vessels from the transverse surface of the block.

Questions 126-184: For questions about plant fibers please refer to the Wayne's Word article about Plant Fibers Used For Paper, Cordage & Textiles. Important plants used for fibers are summarized in the following table:

Stem (Bast) Fibers (Dicots)
Common Name	Scientific Name	Plant Family
Flax	Linum usitatissimum	Linaceae (Flax)
Ramie	Boehmeria nivea	Urticaceae (Nettle)
Jute	Corchorus capsularis	Tiliaceae (Basswood)
Kenaf	Hibiscus cannabinus	Malvaceae (Mallow)
Beach Hibiscus	Hibiscus tiliaceus	Malvaceae (Mallow)
Roselle	Hibiscus sabdariffa	Malvaceae (Mallow)
Urena	Urena lobata	Malvaceae (Mallow)
Sunn Hemp	Crotalaria juncea	Fabaceae (Legume)
Indian Hemp	Cannabis sativa	Cannabaceae (Marijuana)
Indian Hemp	Apocynum cannabinum	Apocynaceae (Dogbane)
Hoop Vine	Trichostigma octandrum	Phytolaccaceae (Phytolacca)
Leaf Fibers (Monocots)
Sisal	Agave sisalana	Agavaceae (Agave)
Henequen	Agave fourcroydes	Agavaceae (Agave)
Yucca	Yucca elata	Agavaceae (Agave)
Abaca	Musa textilis	Musaceae (Banana)
Bowstring Hemp	Sansevieria trifasciata Sansevieria roxburghiana Sansevieria hyacinthoides	Agavaceae (Agave)
New Zealand Flax	Phormium tenax	Agavaceae (Agave)
Seed Fibers (Dicots and Monocots)
Cotton	Gossypium hirsutum Gossypium arboreum Gossypium herbaceum Gossypium barbadense	Malvaceae (Mallow)
Coir	Cocos nucifera	Arecaceae (Palm)
Milkweed	Asclepias spp.	Asclepiadaceae (Milkweed)
Fibers From Seed Pods (Dicots)
Kapok	Ceiba pentandra	Bombacaceae (Bombax)
Floss Silk	Chorisia speciosa	Bombacaceae (Bombax)
Devil's Claw	Proboscidea parviflora	Martyniaceae (Martynia)

More Photos Of Plants Used For Fibers
More Photos Of Plants With Bast Fibers
See Article About The Anatomy Of Wood
See Article About The Anatomy Of Stems
Go To Wood/Plant Fiber Crossword Puzzle

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