Dr. T logo BIO 410/510 Plant Anatomy


•  Support

•  Sclerenchyma provides rigid (elastic) support

•  Most important in mature, non-growing regions.

•  Cells do not fully mature until after growth of the region is complete.

•  Protection

•  Sclerenchyma is common in seed coats and fruit walls

•  May be part of the epidermis or subepidermal layers.

•  Sclerenchyma cells can be found in other tissues

•  Note that sclerenchyma cells can be found in sclerenchyma tissue (where all cells are sclerenchyma) but also in other complex tissues (especially xylem and phloem)

Cell characteristics

•  Secondary wall present

•  Wall thickened, often extremely thick

•  Pits may be present, but usually not numerous

•  Wall thickening uniform (unlike collenchyma)

•  Usually liginified

•  Protoplast usually dead at maturity

Cell types

•  Sclereids

•  Cells are approximately isodiametric

•  Cells are roughly the same size in all dimensions, but can be somewhat cylindrical or have branching projections.

•  Sclereids classified based on shape

•  Brachysclereid (stone cell)

Stone cells
The fruit of Pyrus (pear) contains clusters of stone cells (brachysclereids). Note the thick lignified secondary walls with branching (ramiform) pits.
Micrograph by John Tiftickjian
•  Stone cells sometimes have branching (ramiform) pits.

•  Macrosclereid

Phaseolus sclereids
Cross section of Phaseolus (bean) seed showing two layers of sclereids the seed coat. The outer layer (actually the epidermis) is composed of macrosclereids.
Micrograph by John Tiftickjian
•  Macrosclereids are column shaped. They are commonly found in epidermis and underlying layers of seed coats.

•  Osteosclereid

•  Osteosclereids are bone-shaped, cylindrical with enlarged ends.

•  Astrosclereid/trichosclereid

•  Astrosclereids in Camellia petiole section
Camellia sclereids
Cross section of Camellia petiole showing an astrosclereid. [Micrograph by John Tiftickjian]
•  Astrosclereids (or trichosclereids) in water lily leaf (maceration)
Macerated leaf of Nymphaea showing whole sclereids (astrosclereids).
Micrograph by John Tiftickjian
•  The distinction between astrosclereid and trichosclereid is somewhat subjective. The difference is based on the thickness of the walls of the branches, thick-astrosclereid, thin-trichosclereid. It may be simpler just to call all star-shaped branching sclereids astrosclereids.

•  Fibers

•  Cells many times longer than wide

•  Typical size: 10-20 µm wide, up to 1000 µm (or more) long

•  End walls tapered, needle-shaped

•  Whole cells from macerated wood

Quercus wood fiber
Quercus (oak) wood maceration showing a fiber.
Micrograph by John Tiftickjian

•  Cross section

Fibers, x.s.
This stem cross section of Cucurbita shows perivascular fibers with their thick lignified walls.
Micrograph by John Tiftickjian

•  Longitudinal section

Helianthus stem, l.s.
Helianthus stem, l.s. showing xylem, phloem, and fibers
Micrograph by John Tiftickjian

•  May be septate

Vitis stem, l.s.
Vitis (grape) stem radial section showing phloem on left, xylem on right [Micrograph by John Tiftickjian]

•  Septa are formed when a fiber cell divides after reaching its full length and forming its secondary wall.

•  Septa are seen in longitudinal section as thin transverse walls that divide the fiber into several compartments.

•  May occur in ground tissues

•  For example perivascular fibers in a stem

•  Common in vascular tissues

•  Xylem fibers

•  Wood fibers in secondary xylem
Quercus wood fibres
Cross section of Quercus wood showing fibers and rays.
Micrograph by John Tiftickjian

•  Phloem fibers

•  Fibers are very common in phloem of stems.

Typical locations of sclerenchyma cells and tissue

•  Stem

•  Perivascular fibers in cortex

Cucurbita stem, x.s.
Cross section of Cucurbita (squash) stem showing perivascular fibers.
Micrograph by John Tiftickjian

•  Phloem fibers on outer edge of vascular bundles (bundle "caps")

Helianthus stem, x.s.
Cross section of Helianthus stem. Note ring of vascular bundles, pith, cortex, and epidermis.
Micrograph by John Tiftickjian

•  Leaf

•  Usually associated with veins (fibers)

Phormium leaf, x.s.
Cross section of Phormium leaf. Note large areas of supporting fibers and thin-walled parenchyma cells that function in water storage.
Micrograph by John Tiftickjian

•  Can be present in bundle sheath and bundle sheath extensions

Poa leaf, x.s.
Cross section of Poa leaf. Bundle sheath and bundle sheath extension are composed of fibers. A stoma is visible in the lower epidermis.
Micrograph by John Tiftickjian

•  Fruits and seeds

•  In hard layers like seed coats and stoney fruit pits (sclereids)

•  Rare in roots

•  Support not as important in roots as they are surrounded by soil.

•  Idioblasts

•  In addition to forming definite tissue regions, sclerenchyma cells (sclereids) can be idioblasts-single isolated cells surrounded by cells of another tissue (usually parenchyma). The astrosclereids discussed above are examples of idioblasts.

Economic importance of fibers

•  Different definitions of "fiber"

•  Commercial definition

•  Commercially, fiber can mean any long thread-like structure, natural or synthetic, that can be used for a "fibrous" product.

•  If the "fibers" come from a plant, they may be:

•  True fibers, defined as sclerenchyma cells
•  Other cells
•  Seed coat hairs (actually epidermal tissue)
•  Wood "fiber" (contain fiber cells but also other xylem cells such as tracheids)

•  Fibers used this way can also be materials like nylon and other man-made materials.

•  Botanical definition

•  A true fiber is a type of sclerenchyma cell as we have defined above.

•  Nutritional definition

•  What about getting "fiber" in your diet?

•  This use of the term does not refer to a cell type but to the fibrous nature of cell walls.

•  Here, "fiber" refers to the fibrous nature of cellulose molecules (what we have called microfibrils) and other cell wall polymers (pectin, lignin).

•  Materials made with plant "fibers"

•  Fabrics

•  Rope

•  Paper

•  Brushes

•  Composite materials

•  Fiber cells can be classified on the basis of the tissue they belong to

•  Xylary fibers (wood fibers)

•  Found in secondary xylem

•  Extraxylary fibers (bast fibers)

•  Phloem fibers

•  Cortical stem fibers

•  Leaf fibers

•  Important fiber plants

•  Dicots

•  Linum (flax)

•  Phloem fibers of stem used to make linen.
•  These fibers have little lignification and produce a soft fabric.

•  Cannabis sativa (hemp)

Cannabis stem, x.s.
Cross section of Cannabis stem. Note thick-walled fibers.
Micrograph by John Tiftickjian
•  Phloem fibers of stem used to make rope.
•  Of course, this species also has some other (not so legal) uses :-)

•  Corchorus capsularis (jute)

•  Stem fibers used for rope.

•  Gossypium (cotton)

•  Not true fibers but epidermal hairs (trichomes) of the seed coat used for fabric, and many other uses.
•  Commercially, cotton is the most important fiber product in the world.

•  Ceiba (Kapok tree)

•  Seed coat epidermal hairs used for fabric

•  Monocots

•  Sansevieria (bowstring hemp)

•  Leaf fibers used for rope.

•  Musa textilis (Manila hemp)

•  Related to banana

•  Agave sisalana (sisal hemp)

•  Softwoods (gymnosperm) and hardwoods (dicots) for paper

•  Strictly speaking, paper made from wood pulp is not entirely a fiber product because many of the cells that make up the "fiber" are tracheary elements (water-conducting xylem cells). But such cells are similar in shape and composition to sclerenchyma fibers.

•  Many types of paper can be made depending on:

•  Species the wood comes from
•  Morphologies of cells involved
•  Softwoods are mostly tracheids
•  Hardwoods also contain vessels and fibers
•  Cell wall thickness
•  Length of individual cells
•  Chemical characteristics of cell walls
•  Pulp processing methods

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