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THE TISSUES
-Anatomy is the study of internal structure of plants and other organisms.
-A tissue is a group of cells having common origin and
function.
-Based on the capability of cell division, plant tissues are 2
groups: Meristematic and Permanent.
MERISTEMATIC TISSUES (MERISTEMS)
These are the tissues where active cell division and growth
occurs. Based on the position, meristems are 3 types:
oApical meristems: They occur at the tips of roots and
shoots and produce primary tissues. It is 2 types:
§Root apical meristem: It occupies the tip of a root.
§Shoot apical meristem: It occupies the distant most
region of the stem axis. Some cells ‘left behind’ from
shoot apical meristem, constitute the axillary bud. They
are present in the axils of leaves and can form a branch
or a flower.
oIntercalary meristems: They occur between mature
tissues. They occur in grasses and regenerate parts
removed by the grazing herbivores.
Apical and intercalary meristems are primary meristems
because they appear early in a plant life and contribute to
the formation of primary plant body. During that, specific
regions of the apical meristem produce dermal tissues,
ground tissues and vascular tissues.
oSecondary (lateral) meristems: The meristems that occur
in mature regions of roots and shoots. They are cylindrical
meristems. They are seen in gymnosperms and dicots.
E.g. Fascicular vascular cambium, interfascicular
cambium & cork cambium. These are responsible for
producing the secondary tissues.
PERMANENT (MATURE) TISSUES
-The cells produced by primary and secondary meristems,
become structurally and functionally specialized and lose
the ability to divide. They are called permanent (mature)
cells and constitute the permanent tissues.
-They are 2 types: Simple and Complex.
1. Simple Permanent Tissues
-The tissues having all cells similar in structure & function.
-3 types: Parenchyma, Collenchyma and Sclerenchyma.
a. Parenchyma
-It forms the major component within organs.
-Cells are generally isodiametric. They may be spherical,
oval, round, polygonal or elongated.
-Their walls are thin and made up
of cellulose.
-Cells are closely packed or have
small intercellular spaces.
-Functions: Photosynthesis,
storage, secretion etc.
b. Collenchyma
-It occurs in layers below the
epidermis in dicot plants.
-It is found as a homogeneous
layer or in patches.
-It consists of cells with much
thickened corners due to
deposition of cellulose, hemicellulose and pectin.
-Intercellular spaces are absent.
-Cells are oval, spherical or polygonal and often contain
chloroplasts.
-Functions: They provide mechanical support to the
growing parts such as young stem and petiole of a leaf. The
cells that contain chloroplasts assimilate food.
c. Sclerenchyma
-It consists of long,
narrow cells with thick
and lignified cell walls
having a few or
numerous pits.
-They are usually dead
without protoplasts.
-Based on the form, structure, origin and development,
sclerenchyma is 2 types: fibres & sclereids.
oFibres: These are thick-walled, elongated and pointed
cells, generally occurring in groups.
oSclereids: These are spherical, oval or cylindrical,
highly thickened dead cells with very narrow cavities
(lumen). These are found in the fruit walls of nuts; pulp
of fruits like guava, pear and sapota; seed coats of
legumes and leaves of tea.
-Function: It provides mechanical support to organs.
2. Complex Permanent Tissues
-These are made of more than one type of cells and they
work together as a unit.
-2 types: Xylem and Phloem.
a. Xylem
-It functions as a conducting tissue for water and minerals
from roots to the stem and leaves.
-It also provides mechanical strength to the plant parts.
-It is composed of 4 kinds of elements: tracheids, vessels,
xylem fibres and xylem parenchyma.
