Coupon Accepted Successfully!


Plant Tissues

The Tissues
“A tissue is a group of cells which has its own common origin and performs a common function.”

The formation of tissues in higher plants begins in growing regions called meristems where active mitosis takes place.


Meristematic Tissues

Meristems are groups of cells in the plant body which are in a continuous state of division or retain the power of cell division. The meristematic cells are characterised by the following features:
  1. They are spherical, oval, polygonal or rectangular (isodiametric) in shape; they may be long and narrow in some cases (the fusiform initials of vascular cambium).
  2. Their thin cell walls are made up of cellulose.
  3. They generally lack intercellular spaces.
  4. They do not contain stored food reserves and are in an active state of metabolism.
  5. They are rich in protoplasm and have either numerous small vacuoles or no vacuoles at all.
  6. They have large nuclei and proplastids.
  7. They may have a specific plane of division although some of the meristematic cells may not show this characteristic.
The plant tissues can be conveniently classified into three groups, namely temporary tissues, permanent tissues and secretory tissues.

Temporary Tissues (Meristematic Tissues)

The meristematic tissues are classified into various categories based on their origin, plane of division and function.

Based on their origin meristematic tissues are classified as:

Primary Meristem - It gives rise to permanent tissues of the plant body.

Based on their position it is classified into:
  1. Apical Meristem
  2. Intercalary Meristem
  3. Lateral Meristem
Apical Meristems
Present at the apices of stems (shoot), roots, branches and leaves.

Lateral Meristems
Present along the lateral sides of dicot stems and roots e.g. Cork cambium.

Intercalary Meristems
Present between permanent tissues e.g. Grass and at the nodal regions.

Secondary Meristems
It represents actively dividing groups of cells which develop from permanent tissues which have already undergone differentiation. Phellogen or cork cambium and cambium of dicot stems and roots are examples of secondary meristems.

Based on the plane of division meristematic tissues are grouped into mass meristems, plate meristems and rib meristems.

Functionally meristems are classified into protoderm, procambium and ground meristem. Protoderm is the outermost layer of cells, one cell thick, which develop into the epidermis. Cells of procambium appear as small patches in the developing stem or root and retain the capacity for cell division. They give rise to permanent vascular tissues.

Fig: Plant tissues as found in seeds, stems, and roots of higher plants. Meristematic tissues are present in the meristem of buds and roots and in the cambium layer between the phloem and xylem.

Permanent Tissues

Permanent tissues usually do not change into other kinds of tissues, but in most cases they retain their structural and functional characteristics throughout life. Permanent plant tissues can be grouped into two main categories: simple tissues and complex tissues.

Simple tissues
Simple tissues are composed mostly of one kind of cell and all are constructed similarly. There are various types of simple tissues:
  1. Epidermis
    Usually one cell thick and colourless, but such pigments as blue, red or purple may be present, outer cell walls are coated by a waterproof, waxy cutin; however, guard cells that control gas movements through epidermal pores called stomata, possess chlorophyll in bodies called chloroplasts.

    Epidermal tissues are found on the surface of leaves, flower parts and younger stems and roots. They function as protective layers against mechanical injuries and the effects of parasites. They also assist in conserving internal moisture.
  2. Parenchyma
    Cells are usually void or spherical, but sometimes cylindroid and with a large central vacuole; cell walls are thin; usually with numerous intercellular spaces; protoplasm may remain alive for a long time; parenchyma cells may form tissues by themselves, but they may be mixed with other types of cells in complex tissues.

    Parenchyma is very abundant in higher plants. Parenchyma (pith) of roots, stems and fruits stores water and food (potato tubers, tomato pulp, watermelon etc.).
  3. Collenchyma
    They are elongated cells with unevenly thickened walls due to the presence of cellulose, hemicellulose and pectin. They are present in the celery petiole. The cell can assimilate food with chloroplast.
  4. Sclerenchyma
    Cell walls are greatly thickened with cellulose and lignin to give strength and rigidity (corn stem), Protoplasm is not long-lived. These are
    1. fibres which are tough, pliable and strong. The cells are elongated with tapering ends
    2. sclereids or stone cells in which cells are not elongated but give strength and support (the hull of walnuts).
The sclerenchyma tissues give mechanical support and strength and because of the cohesive powers of the cells they can be used to make threads, ropes and textiles. Two types of sclerenchyma are found in plants: 

  1. Fibres which are tough, pliable and strong. The cells are elongated with tapering ends.
    Fibres: They are common in plant stems, sclereids are common in shells of nuts and are found as gritty masses in pears etc.
  2. Sclereids or stone cells in which cells are not elongated but give strength and support (the hull of walnuts)

  1. Cork
    Cells usually are rectangular and regularly arranged; cell wall contains waterproof waxy material called suberin; protoplasm dies rather soon.
    1. These tissues are found in the outer bark of stems and roots of woody plants where it serves as a protective layer against mechanical injuries and excessive loss of internal moisture (cork oak tree).

Complex tissues

Complex tissues consist of several types of cells that usually are engaged in a group of closely related activities. The two main complex tissues found in plants are xylem and phloem.
  1. Xylem
    These are composed of
    1. Tracheids which are elongated, tapering cells with short - lived protoplasm; cell walls usually are strengthened and thickened by spirals or rings of lignocellulose and often with thin areas (pits);
    2. Vessels which are long, rather large, tubular structures made of a series of cells whose walls may be thickened and have pits. Other tissues in xylem may include fibres, xylem parenchyma, and ray cells.
Xylem tissues give strength and conduct water, dissolve mineral salts, and sometimes foods upward through stems (also laterally). Tracheids are the chief conducting tissues in gymnosperms (Pinus); xylem vessels are the chief conducting structures in angiosperms (Oak).

  1. Phloem
    Phloem consists of two types of cells:
    1. sieve tubes,
    2. companion cell
    3. phloem parenchyma and
    4. phloem fibres
  1. Sieve Tubes
    They are elongated rows of cylindroid cells whose end walls contain sieve plates containing numerous pores like those in a sieve; protoplasm remains alive. Higher plants' sieve cells lack a nucleus at maturity, but some lower vascular plants are exceptional. Tonoplast is often not discerned at functional maturity.
  2. Phloem Parenchyma
    The cells of these tissues have characteristics as described under parenchyma. Other tissues in phloem include fibres and ray cells. In flowering plants, elongated, living, nucleate cells known as companion cells, sieve tubes, which possibly assist in conduction or in food storage.
Phloem tissues give strength and mainly conduct foods downward from leaves through stems and roots; phloem parenchyma stores food.

Secretory Tissues

The tissues or cells involved in production of secretions or excretions in plants constitute the secretory tissues. The chemicals produced by plant cells for their own use are called excretions, e.g. enzymes and hormones. The chemicals produced by plant cells that are of no use to plants, but are useful to man, are called secretions, e.g. opium, asafoetida, rubber, resins, gums and oils. The tissues engaged in the production of secretions and excretions are of two types: lactiferous tissues and glandular tissues.
  1. Lactiferous Tissue
    This type of tissue is composed of living, thin-walled, elongated, branched and multinucleate tube-like structures containing colourless, milky or coloured juice, called the latex. These specialised cells are scattered in the ground tissue of the plant body and contain organic materials in the form of starch, rubber, tannins, alkaloids, enzymes and proteins. The tissue represents two types of structural components called latex vessels and latex cells. The functions of laticiferous tissue differ in different species. In Euphorbia it stores starch; in certain plants it contains poisonous alkaloids to protect the plants from grazing animals and in Hevea it contains commercial rubber.
  2. Glandular Tissue
    A gland is a group of cells specialised for secreting or excreting a specific product. The cells constituting the glands are living, parenchymatous with thin cellulose walls. The glands in plants may be external or internal. External glands occur external to the epidermis of stems, leaves or floral parts (e.g. glandular hair, nectarines and enzyme secreting glands). The digestive glands of insectivorous plants produce protein and digestive enzymes on the general surface or through glandular hair. Internal glands in plants are embedded in various tissues of roots, stems, leaves, flowers and fruits. In Pinus, resin ducts which secrete resins occur embedded in protoxylem, secondary xylem and in the cortex. The leaves of Eucalyptus have glands which secrete aromatic oils. Hydathodes are specialised groups of cells present at the vein endings of leaves. These are also known as water secreting glands as they lose water in the form of liquid droplets.

Test Your Skills Now!
Take a Quiz now
Reviewer Name