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Summary BOT 2601 LEARNING UNIT 9
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- University Of South Africa (Unisa)
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By: marisavanpletzen • 2 year ago
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BOT 2601 LEARING UNIT 9FIBRES, FIBRE PRODUCTS AND FORAGE FIBRES
Textbook reference: chapter 12 pg. 399-419
EXTRAXYLARY FIBRES:
o True fibres are divided into xylary fibres and extraxylary fibres
o Xylary fibres are found in the secondary xylem and are classified by pitting and wall characteristics
o Extraxylary fibres are classified by their site of origin or location in the plant body.
o Fibres positioned in the phloem or cortical regions of the stem of dicotyledons are strong and flexible. They are
used in the manufacture of cloth, linen and threads
o Lignification results in an undesirable increase in the crispness and a decrease in flexibility of the fibre bundles.
This decreases fibre quality
XYLARY FIBRES AND PAPER MANUFACTURE:
o Wood is the raw material in paper
o The mechanical and chemical conversion of wood into pulp consumes vast quantities of timber
o Paper is manufactured from the secondary xylem of softwood and hardwood
o Wood pulp is composed of fibres
o Pulp made from softwoods is homogeneous and composed of long tracheids
o Pulp made from hardwood is heterogeneous and composed of wide vessel elements and narrow thick-walled
cells that are true fibres
o Softwoods produce coarse, strong pulps
o Hardwoods produce fine smooth papers
o Wood fibre properties effecting quality of paper: fibre length, tensile strength, cross-sectional shape, wall
thickness, elastic modulus (flexibility) and surface chemistry
o Paper is made from cell-sized particles, continuously formed wet into thin sheets and dried over hot rolls. 1 st in
the manufacturing process is the maceration of wood, resulting in separation of component cells to form fibres
o The macerated xylem is known as wood pulp
o The process of chemical pulping removes lignin and includes bleaching, beating and refining
o Pulp fibres are integrated into a web that forms paper
o Wall hemicellulose helps form fibre-to-fibre bonds in the papermaking process that helps strengthen paper
o It is important to remove lignin to decrease fibre stiffness
REACTION WOOD:
o The main axis or lateral branches of actively growing stems are stimulated to form wood with a growth pattern
that is composed of cells with an atypical structure and chemical composition
o This secondary xylem is reaction wood and it forms along one side of the displaced axis in response to gravity
o The amount of reaction wood formed by the vascular cambium is correlated with the degree of axis lean from
the vertical upright position
o Trees can be stimulated to form reaction wood in main stems deviating as little as 2 degrees from the vertical
o The development of reaction wood is an auxin-mediated response
o Reaction wood does not form in monocots
o Reaction wood in conifers is compression wood
o Reaction wood in dicots is tension wood
o Compression wood: also known as redwood
- Forms on lower or compression side of the woody cylinder
- Appears as eccentric layers of dense, reddish- or yellowish-brown wood and has wide growth layers
- In response to changes in moisture content, reaction wood shrinks and swells more than normal wood does
during the seasoning process
- During drying, lumber of reaction wood becomes bowed and warped due to abnormal longitudinal shrinkage
- Tracheids are shorter, thicker walled, heavily lignified and rounded
o Tension wood and gelatinous fibres:
- Forms along the upper side of leaning stems and branches
- Presence of gelatinous or mucilaginous fibres
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