Sclerenchyma Cells and Tissues
Sclerenchyma cells or tissues are variable in appearance but
one key attribute of all types is the thick secondary wall. In longitudinal
sections sclerenchyma cell types are more challenging to identify but wall
characteristics still apply.
Sclerenchyma cells provide protective, supportive, and vascular
functions. The protective/supportive sclerenchyma include fibers and
sclereids. The vascular sclerenchyma is xylem.
Vascular tissues of plants contain xylem and phloem.
Xylem cells usually stand out as sclerenchyma cell types whereas phloem cells
blend in with other parenchyma. The next two units present more information
about xylem and phloem. These are typically found side by side in vascular
tissues.
Now, examine this image closely. Sclerenchyma cells in this
view are fibers or xylem. Note the thick secondary cell walls and large lumens
of the xylem cells to the right. Contrast these to the smaller diameter fiber
cells with thick walls to the upper left. You should note most sclerenchyma
cells are dead at maturity and therefore lack nuclei.
Now, roll over the fiber immediately above
the top central xylem vessel.
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Fibers
Fibers are long, durable cells that strengthen and support plant
structures. Plant fibers of different types can be of economic value. For
example, long fibers of cotton or jute are used to make rope while short fibers
of woods are used to make papers of various types.
In this view of oak wood, fibers and xylem are both visible
in longitudinal view. Long, thinner cells are fibers. Length and rigidity
suggests fibers act as microscopic "reinforcing rods" in plant tissues.
Xylem cells in this section are larger and appear to possess openings or pores.
Pores are adaptions that enable xylem to conduct water efficiently.
Most plants need support and therefore possess fibers within
their tissues. Look for these near vascular elements and in stems just below
the epidermis. Most plants also require vascular tissues and therefore contain
xylem.
Now, roll your mouse over the longitudinal
fibers just visible in the top, right corner of the image.
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Comparison of Fiber and Xylem
In a side-by-side cross-section comparison of fibers and xylem
you can see a fundamental difference between these two sclerenchyma cell
types. Remember, fibers are long, thin, pointed cells that serve in support.
In cross sections like this their most obvious features are thick secondary
walls and a narrow lumen. Fibers tend to be more uniform in size and shape
whereas xylem cells can be variable.
Now, let's examine sclereids. Roll over
the intercellular space to the right of the fiber in the upper right corner
of the image.
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Sclereids
In contrast to fibers, sclereids are short, irregularly shaped
cells. In common with fibers, sclereids possess thick secondary walls. Look
at the smaller, more uniform cells at the top. These are fibers. Below these
is a large sclereid. Note the irregular shape.
Sclereids continue expanding their secondary walls to the
point where cytoplasmic space is very limited. Plasmodesmata play a role
in enabling these cells to remain alive as the secondary walls thicken.
Look for these here.
Sclereids can be found in soft tissues of plants. Here, they
may act as defensive cells by rendering the tissues unpalatable to chewing
insects. The gritty texture of pears is due to sclereids.
Sclereids also form the hard almost impenetrable coverings
of some nuts. They are also abundant in the coats of seeds, capsules, and
other plant structures.
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