Parenchyma is the most common and versatile
ground tissue. It forms, for example, thecortex and pith of stems, the cortex of roots, the mesophyll of
leaves, the pulp of fruits, and the endosperm of
seeds. Parenchyma cells are living cells and may remain meristematic at
maturity, meaning that they are capable of cell division. They have thin but flexible cellulose cell walls, and are generally polygonal when
close-packed, but approximately spherical when isolated from their neighbours.
They have large central vacuoles, which allows the cells to
store and regulate ions, waste products and water.
Parenchyma cells have a variety of functions:
§
In leaves,
they form the mesophyll and
are responsible for photosynthesis and the exchange of gases[1], parenchyma cells in themesophyll of
leaves are a specialized parenchymatous tissue known as chlorenchyma (parenchyma
with chloroplasts).
§
Storage of starch, protein, fats
and oils and water in roots, tubers (e.g. potato), seed endosperm (e.g. cereals) and cotyledons (e.g.pulses and groundnut)
§
Secretion (e.g. hydathodes, nectaries and
cells lining the inside of resin ducts)
§
Wound repair and the potential
for renewed meristematic activity
§
Other specialized functions such
as aeration (aerenchyma) and support
The form of parenchyma cells varies with
their function. The epidermal parenchyma cells of a leaf are barrel shaped in
cross section, but have a variety of outline shapes ranging from simple
polygons to strongly branched and interlocked shapes resembling the pieces of a jigsaw puzzle, as in the leaves of Arabidopsis thaliana.
In the epidermis of higher plants, only the guard cells have chloroplasts. This tissue serves as a barrier
wall and protects the internal tissues from injury[1]. In the spongy mesophyll of
a leaf, parenchyma cells range from near-spherical and loosely arranged with
large intercellular spaces[1] to
branched or stellate, mutually interconnected with their neighbours at the ends
of the arms to form a three-dimensional network, as in the red kidney bean Phaseolus vulgaris and other mesophytes.[2] These
cells, with theepidermal guard cells of
the stoma,
form a system of air spaces and chambers that regulate the exchange of gases.
They usually contain plastids .[1].
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