PLANT TISSUES

DEFINITION OF TISSUE: A Tissue is a group of similar cells to perform a specific function.

TYPES OF TISSUES:

1. MERISTEMATIC TISSUE OR MERISTEM

    (a) NATURE:  This consists of cells that have the power of dividing.

                             The cells are oval, spherical or polygonal.

                             The cell wall is thin and is made up of cellulose.

                             The cells are compactly arranged as there is no intercellular space.

                             Each cell contains a large nucleus.

                             The cytoplasm is dense, hence vacuoles are reduced.

     (b) LOCATION AND FUNCTION:  According to their position in plants, Meristematic tissues are classified:

                                  APICAL MERISTEM—are situated at the growing tips of stems, roots and their branches.

                                  These bring about elongation of root and stem.

                                  LATERAL MERISTEM—are found beneath the bark and responsible for the increase in the

                                  diameter of the stem.

                                  INTERCALARY MERISTEM—are located at the base of the internodes and at the leaf base.

                                  This helps in elongation of internodes and leaves.

2. PERMANENT SIMPLE TISSUE:  This Tissue is made up of similar permanent cells having the same function.

    It is of following types:

1. PARENCHYMA:

NATURE:  It consists of living cells that are equally expanded on all the sides.

                   They are generally spherical or oval.

                   Cell wall is made up of cellulose.

                   It encloses cytoplasm which contains small nucleus surrounded by large vacuoles.

                   Intercellular spaces are plenty.

LOCATION:  It is found in the soft part of stem, roots and leaves such as cortex of roots, ground tissue in stem

                       and mesophyll in leaves.

FUNCTION:  They store food and provide temporary support to the plant.

NOTE: When parenchyma contains chloroplast then it is called chlorenchyma and perform photosynthesis and

             When Parenchyma contains air spaces, then it is called aerenchyma and perform the diffusion of

             respiratory gases.

2. POLENCHYMA:

NATURE:  It also consists of living cells.

                   The cells are comparatively longer and broad with rounded ends.

                   The cell walls are unevenly thickened with cellulose.

                   Intercellular space are generally absent.

                  LOCATION:  It is found below the epidermis of dicotstems  and petiole of leaves.

                  FUNCTION:  It gives support to the plant and if it contains chloroplast, it manufactures food.

           (c )  SCHELERENCHYMA:

                  NATURE:  It is the hardest plant tissue.

                                     On maturity, the cells of schelerenchyma are dead.

                                     It consists of very long, narrow cells tapering at the ends to resemble fibres.

                                     These cells are of two types—Schelerenchyma fibres and sclereibs(stone cells).

                 LOCATION:  It is found in stems, veins of leaves, hard covering of seeds and nuts and pulp of pears.

                 FUNCTION:  It is protective and gives rigidity, strength, flexibility, elasticity to the plant body.

     3. CONDUCTING TISSUE:  It is of following types:

           (a) XYLEM:  Parts of xylem tissue are:

                (i) TRACHEIDS:  These are elongated dead cells with large cavities. They provide mechanical support.

                (ii) XYLEM VESSELS:  They are broader as compared to tracheids and are joinedend to end with their walls

                       having small poresto give it a tube like appearance.

                (iii) XYLEM FIBRES:  They are schelerenchyma fibres present in xylem and provide mechanical support.

                (iv) PARENCHYMA:  These cells are small, thick walled, store food and helps in sideways conduction of water.

                 LOCATION:  Xylem is found in all parts of the plant such as root, stem and leaves.

                 FUNCTION:  Xylem tissues conducts water and minerals from the root to leaves.

1. PHLOEM:  Parts of pheolem tissue are:

(i)COMPANION CELLS:  They are narrow, elongated cells that controls the activity of narrow sieve tubes.

                  (ii) PHEOLEM PARENCHYMA:  Parenchyma occurring along with pheolem is known as pheolem

                        parenchyma. these cells store food and take part in the sideward conduction of food.

                  (iii) PHEOLEM FIBRES: Schelerenchyma fibres found in pheolem are known as pheolem fibres. They provide

                         mechanical strength.

                  (iv) SEIVE TUBES: They are tube like channels which are formed by end to end arrangement of a number of

                         cells. They bear a number of small pores and the end walls are also called sieve plates. These plate connect

                         the protoplasm of the adjacent channels to produce long tube channels.

                 LOCATION:  It is present in all parts of plant, i.e., leaves, roots, stems.

                 FUNCTION:  It conducts food from leaves to the other parts of the plant.

                 NOTE: Xylem along with phloem constitutes vascular bundle.