Download Parenchyma and collenchyma tissues The term parenchyma refers

Dr.Muhannad.M.Sahib
Plant histology (lecture4)
College of Biotechnology
Qadysia university
Parenchyma and collenchyma tissues
The term parenchyma refers to a tissue composed of living cells variable in their morphology
and physiology, but generally having thin walls and a polyhedral shape (Fig. 7.1), and
concerned with vegetative activities of the plant. The individual cells of such a tissue are
parenchyma cells. The word parenchyma is derived from the Greek para, beside, and enchein, to pour, a combination of words that expresses the ancient concept of parenchyma as a
semiliquid substance “poured beside” other tissues that are formed earlier and are more solid.
Inasmuch as the presumed ancestors of plants consisted entirely of parenchymatous cells
parenchyma may be considered the phylogenetic precursor of all other tissues. This tissue is
the principal seat of such essential activities as photosynthesis, assimilation, respiration,
storage, secretion, and excretion—in short, activities depending on the presence of complete
living protoplasts. Parenchyma cells that occur in the xylem and phloem play an important
role in the movement of water and the transport of food substances.
Distribution:
Parenchyma Cells May Occur in Continuous Masses as Parenchyma Tissue or Be
Associated with Other Cell Types in Morphologically Heterogeneous Tissues.
Examples of plant parts consisting largely or entirely of parenchyma cells are the pith and
cortex of stems and roots, the photosynthetic tissue (mesophyll) of leaves the flesh of succulent
fruits, and the endosperm of seeds. Parenchyma cells form the vascular rays and the vertical
files of living cells in the xylem and phloem.
Origin:
These basically arise from primary meristematic tissue and form the framework of all plant
organs. Parenchyma of cortex ,pith ,mesophyll of leaf and of floral parts arise from ground
meristem and that found in primary vascular tissues arise from ground meristem and that
found in primary vascular tissues arise from the procambium. During secondary growth, they
also arise by activity of cambium and cork cambium.
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Dr.Muhannad.M.Sahib
Plant histology (lecture4)
College of Biotechnology
Qadysia university
Structure:
For studying structure of parenchyma some important points are emphasized i.e., shape,
arrangement and cell contents.
i-
Shape
Parenchyma cells are usually isodiametric but they vary in form. In several cases,
these become elongated and merge with prosenchyma cells in shape and dimensions.
(The term prosenchyma was earlier used for elongated cells with tapering ends but
in view of the fact that different types of cells may be elongated, the term has gone
out of use.) Parenchyma of mesophyll tissues are variously lobed, folded and armed.
In general, the parenchyma cells ideally have 14 facets or sides.
ii-
Arrangement
Mature parenchyma cells are either packed closely or with small intercellular spaces.
The parenchyma of endosperm which serves as storage tissue usually lack
intercellular spaces but those of fleshy fruits which are also storage parenchyma have
large and numerous intercellular spaces. However, there is always a great variation
as regards to arrangement of parenchyma in the plant body.
iii-
Cell contents:
a- Cell wall: parenchyma usually have thin primary wall composed of cellulose, calcium
pectate and hemicellulose.
b- The cell contents show variation particularly in respect to their activities, i.e.,
photosynthetic cells contain chloroplasts and starch, storage parenchyma contain
ergastic substances like starch grains, protein granules, oil globules, sugar.
Important Functions of parenchyma:
1- These are the centers of various physiological activities like photosynthesis,
assimilation, respiration, storage, secretion, excretion.
2- Parenchyma cells of xylem and phloem in the form of trachea and sieve elements help
in upward and downward conduction of water and dissolved food materials.
3- These are helpful in wound healing.
4- These retain the power of cell division.
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Dr.Muhannad.M.Sahib
Plant histology (lecture4)
College of Biotechnology
Qadysia university
5- Cutinized parenchyma of leaf epidermis are protective in nature which prevents
transpiration and up to some extent mechanical injuries.
6- Parenchyma of cambial cells divide and form secondary tissues.
7- In succulent plants, these help in water storage.
8- In aquatic plants, these store air (aerenchyma), provide buoyancy to float easily in or on
the water.
9- Parenchyma cells of meristems are helpful in the formation of adventitious buds and
roots.
Collenchyma tissue:
Collenchyma are living tissues composed of more or less elongated cells with thick primary
non lignified walls.
Origin:
These are derived from apical meristem and cortical collenchyma and procambium.
Structure:
Collenchyma do not show much variation in their structure.
i-
Cell shape: the shape of cells shows variation. The shorter collenchyma cells are
more or less like parenchyma while the longer ones resemble the fibers with tapering
ends often overlapped. Collenchyma are usually polygonal in cross sections.
ii-
Cell wall: cell wall is the most distinctive feature of collenchyma which is
characteristically unevenly thickened. The wall thickening is primary in nature. It is
composed of cellulose, hemicellulose and pectic materials with high percentage of
water.
iii-
Cell contents: these are living tissue with vacuolated protoplast. Each cell is
uninucleate which may or may not have chloroplast. Collenchyma are closely related
to parenchyma but have thicker walls and longer cells.
On the basis of thickening on cell wall and arrangement of cells, three forms of
collenchyma are recognized into:
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Dr.Muhannad.M.Sahib
Plant histology (lecture4)
College of Biotechnology
Qadysia university
a- Angular collenchyma: the thickening are primarily at the corners or angles of the cells.
As a result, this collenchyma becomes a compact tissue consisting of irregularly
arranged cells without intercellular spaces, as in stems of Datura and Lycopersicon.
b- Lacunar or tubular collenchyma: the thickening are restricted to the walls of the
regions bordering air spaces. These have large intercellular spaces. Such types of
collenchyma are found in Salvia, Malva.
c- Lamellar collenchyma: the thickening occurs chiefly on the tangential walls and lesser
in the radial walls. As a result, cells appear like plates, bands or lamellae. Such
collenchyma are found in the stems of Raphanus and Sambucus.
Distribution: collenchyma are characteristically found in the hypodermis of herbaceous dicots
like Helianthus, Cucurbita, Datura. These occur either as a homogenous layer as in
Helianthus stem or in Patches as in Cucurbita stem. In leaves, collenchyma are found on
either sides of vein as in Mangifera or several other dicot leaves.
Functions: following are some important functions of collenchyma.
1- These are effective mechanical tissues coupled with considerable elasticity which give
support to the growing organs.
2- These provide tensile strength to the growing organs during development.
3- The closely packed cells with thick walls have the capacity of increasing in surface and
in thickness when the organ is still growing.
4- In leaves, these provide support as they occur on both sides of bundles or in isolated
patches.
5- These protect the vascular bundles of leaves by forming cap or bundle sheath like
structures.
6- Collenchyma which have chloroplasts perform photosynthetic functions.
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