Functions

•  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 sometimes have branching (ramiform) pits.

•  Macrosclereid

•  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

•  Astrosclereids (or trichosclereids) in water lily leaf (maceration)

•  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

•  Cross section

•  Longitudinal section

•  May be septate

•  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

•  Phloem fibers

•  Fibers are very common in phloem of stems.

Typical locations of sclerenchyma cells and tissue

•  Stem

•  Perivascular fibers in cortex

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

•  Leaf

•  Usually associated with veins (fibers)

•  Can be present in bundle sheath and bundle sheath extensions

•  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 soft fabric

•  Cannabis sativa (hemp)

•  Phloem fibers of stem used to make rope.

•  Of course, this plant has 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

Delta State University > DSU Sciences > Dr. Tiftickjian > Plant anatomy > Sclerenchyma