Summary
A Tissue
is a group of cells that are alike in origin, structure and function. There are
two principal groups: (1) Meristematic tissues and (2) Permanent tissues.
Meristematic tissues comprise of self- perpetuating cells. Meristems are classified
into several types on the basis of position, origin, function and activity.
Many anatomists illustrated the root and shoot apical meristems on the basis of
the type and arrangement and accordingly proposed many theories. The permanent
tissues normally develop from apical meristem. They are classified into two
types: 1)Simple permanent tissues and 2)Complex permanent tissues. Simple
tissues are composed of a single type of cells only. It is of three types: (1)
Parenchyma (2) Collenchyma and (3) Sclerenchyma. A complex tissue is a tissue
with several types of cells but all of them function together as a single unit.
It is of two types – xylem and phloem. Secretory tissues produce different
types of chemicals. Some are in the form of enzymes, hormones, rubber, gum etc.
The
tissues can be classified on the basis of their function, structure and
location into epidermal tissue system, ground tissue system and vascular tissue
system. Epidermal tissue system develops as the outermost covering of the
entire plant body. It consists of epidermal cells and associated structures.
All tissues except epidermis and vascular tissues constitute the ground tissue.
The vascular tissue system is formed of vascular bundles.
In the
primary structure, the outermost layer of the root is called piliferous layer.
Cortex consists of only parenchyma cells. All the tissues present inside
endodermis comprise the stele. In dicot (Example: bean) root, xylem is tetrach.
Its phloem patch consists of sieve tubes, companion cells and phloem
parenchyma. In monocot (Example: maize) root, xylem is polyarch.
In dicot
(Example: sunflower) stem, stele is eustele type and its vascular bundles are
wedge shaped, conjoint, collateral, open and endarch. In monocot stem (Example:
maize) vascular bundles are scattered and skull shaped, conjoint, collateral,
closed and endarch.
In dicot
(Example: sunflower) and monocot (Example: grass) leaves vascular bundles are
conjoint, collateral and closed.
Hydathodes
discharge liquid water with various dissolved substances from the interior of
the leaf to its surface. Plants that grow in salty environment are called
halophiles. Salt glands typically are found in halophytes.
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