In multicellular plants, tissues are broadly divided into two types – meristematic tissues and permanent tissues.
Meristematic Tissues
Meristematic tissues are a group of actively dividing
cells, mainly located at the growing tips of the roots, stem and leaves.
Types of Meristematic
Tissues
Based on its
location, meristematic tissues are of three types:
Apical meristem It is found at the tips of the shoot and
the roots.
Lateral meristem It is found beneath the bark and the
inner portions (vascular bundles) of the shoot and roots.
Intercalary meristem It is found at the base of the
internodes.
Structure of
Meristematic Tissues
Cells of meristematic tissue are small in size. They are
spherical, longitudinal or oval in shape.
Each cell has a very thin cell wall.
They are compactly arranged without any intercellular spaces.
Each cell contains a large and conspicuous nucleus surrounded by abundant cytoplasm. Generally, vacuoles are absent in meristematic tissues.
Functions of Meristematic Tissues
The apical meristem produces new buds and leaves on the tip
of the stem.
The lateral meristem divides and causes an increase in the
diameter of the roots and shoots of a plant.
The intercalary meristem divides and produces leaves and leaf
sheaths of monocotyledonous plants.
Permanent Tissues
Permanent tissues develop from the new cells formed by the
division of meristematic tissues. The cells of permanent tissues are larger
than the meristematic tissues and have a definite form and size. The cells of
permanent tissues have very little or no protoplasm and have large vacuoles.
They are completely dead cells and adapted to perform some special functions.
Permanent tissues can be divided into two groups- simple
permanent tissues and complex permanent tissues.
Simple Permanent
Tissues
These are composed of similar types of mature cells which
provide support and protection to the plant. The supporting tissues comprise
the three types of simple permanent tissues – parenchyma, collenchyma and
sclerenchyma.
Parenchyma
These are the most common types of plant tissue that are made up
of various plant organs such as roots, stems, leaves and flowers. The
epidermis cells which form the outermost covering of various plant parts are
composed of parenchyma cells. Each parenchyma cell is living and consists of a
nucleus and a large vacuole. The parenchyma cells are round in shape and joined
together with intercellular spaces.
Functions
They store food and water in the roots and stem of the plant.
Parenchyma cells help in the conduction of water in the roots and
stem of the plant.
When they contain Chloroplasts, they help in the manufacture
of food and are called chlorenchyma.
Collenchyma
Collenchyma cells are found in the form of bands in
dicotyledonous stems, petioles of leaves and flower stalks. The collenchymas
cells are circular; oval or polygonal in shape, and are living in nature. They
are joined together without any intercellular spaces. They are thickened at the
corner due to the deposits of cellulose and pectin.
Sclerenchyma
Sclerenchyma cells are found in the epidermis and vascular
bundles of dicotyledonous plants. The mature sclerenchyma cells are usually
dead. There is no intercellular space present between them. Their cell walls
are thickened due to lignin deposits. Simple pits are present in the walls of scelerenchyma cells.
Complex permanent
tissues
These are made up of more than one type of cell. Their main
function is to conduct water and food materials to the different parts of the
plants. Therefore, they are also called conducting tissues or vascular tissues.
Complex permanent tissues are of two types one is the xylem the one is the phloem.
Xylem
The xylem tissues are found in roots, stems and leaves. They
conduct water and dissolved minerals from the root to the shoot system of a
plant.
The xylem consists of four types of cells – tracheids, vessels,
xylem parenchyma and xylem fibres.
Tracheids are long, spindle-shaped
dead cells with thick cell walls.
Vessels Vessels are tube-like dead cells.
They are placed one above the other and form a long tube-like structure through
which water and dissolved minerals are conducted from the roots to the leaves.
Xylem parenchyma These are the only living components
of xylem tissues. They conduct water and store food for the plant.
Xylem fibres These are sclerenchymatous fibres
connected with tracheids and vessels. They are dead cells with thick cell
walls. They provide strength to the xylem tissue.
Phloem
Phloem is found in all parts of the plant. It is made up of
living tubular cells that provide passage for the downward movement of food
prepared by the leaves to other parts of the plant.
The phloem consists of sieve tubes, companion cells, and
phloem parenchyma and phloem fibres.
Sieve tubes are long, slender cells
connected end-to-end and form a tube-like structure that conducts the food
material throughout the plant.
Companion cells are located outside and
alongside the sieve tubes and help in the absorption of the material to be
transported in plants.
Phloem parenchyma These are the parenchymatous cells
associated with the phloem tissue.
Phloem fibres are the dead parts of the
phloem tissues.