b)
Fundamental Tissues
Most of the fundamental tissues
are simple tissues (i.e. each is usually composed of a single type of cell).
They are often defined as those tissues that are neither surface
tissues nor vascular tissues. Examples of fundamental tissues are; the parenchyma cells of
the pith and cortex, groups of collenchyma cells in the angles of stems, and
groups of sclerenchyma cells which form a sheath on the outside of vascular
bundles.
i) Parenchyma tissue is
found in roots, stems and leaves. Parenchyma
cells are relatively unspecialized, like those that make up almost the whole
of the body structure in non-vascular plants. They have not lost the capacity
for cell division and in some circumstances they may take on meristematic
activity or undergo further specialization and form other cell types. They are
alive at maturity.
Parenchyma cells usually have thin primary walls
of cellulose. They generally have single, large vacuoles surrounded by a
peripheral layers of cytoplasm. The
cells are loosely packed and consequently intercellular spaces are abundant. In the leaves, the chloroplasts are located in parenchyma
tissue (which is called chlorenchyma).
Parenchyma of stems and roots functions in the storage of
carbohydrates, nutrients and water. When
turgid, parenchyma is important in providing support and shape to the plant.
ii) Collenchyma cells remain
alive during most of their functional existence. They are structurally similar
to parenchyma cells, except that they are more elongate and have thick primary
cell walls. The latter consist of irregular thickening of cellulose
deposited on the inside. The
thickened areas are usually most prominent at the edges (the
"corners" when viewed in cross-section).
Collenchyma functions as an important support tissue where flexibility
is important, such as in young plants, in the older stems of non-woody plants
and in leaves.
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iii) Sclerenchyma is a
simple type of fundamental tissue that like collenchyma, functions in support.
However, sclerenchyma cells are far more specialized than collenchyma
cells. At functional maturity,
most are dead, and their uniformly very thick, heavily lignified secondary cell walls give strength to the plant body.
Often these walls are so thick that the lumen
(internal space) of the cell has been almost obliterated.
In this picture of a Spartina sp. leaf xs you can see the sclerenchyma
cells as small dark red stained cells just under the epidermis and around the
vascular bundles.
Sclerenchyma cells are customarily divided into two categories according to their shapes. Fibres are very elongated cells with tapered ends. They are stronger and but somewhat less flexible than collenchyma tissues. The fibres from the stems of flax and hemp plants are extracted to make textiles and rope. Sclereids are of variable shapes.
The
simple, unbranched sclereids are frequently called "stone cells".
They are common in nut shells, the hard parts of seeds and scattered
throughout the flesh of soft fruits, such as pears (giving the characteristic
gritty texture).
Another common
shape for sclereids is the star-shaped astrosclereid.
These give support to the flexible, floating, leaf blades of aquatic
plants, such as Nymphaea sp. the water-lily.
iv) Endodermis is a
fundamental tissue because it is the modified innermaost layer of the cortex.
It is found as a layer which surrounds the vascular core in the root
and, less frequently, in the stem. Young
endodermal cells are much like elongate parenchyma cells, except that a band
of thickening runs around their radial (side) and transverse walls. The band consists of lignin
(for strength) and suberin (for
water-proofing) and is called the Casparian
strip. This strip is
continuous in each endodermal cell and is not permeable to water. Therefore, the endodermal cells can regulate the passage of
water and solutes into the plant body. They
do this by directing water and solutes to pass through the cell membranes and
through the bodies of the cells. This
replaces the more usual, passive route for water through cell walls and along
their surfaces.
