Permanent Tissues

About Permanent Tissues type of Permanent Tissues

The tissue whose cells have lost their divisional capacity and have assumed a definite shape and size in response to varied mechanical and physiological functions, is called permanent tissue. .

On the basis of constituent cells, permanent tissues are classified into three groups: Simple tissues, Complex tissues and Special tissues or secretary tissues.

SIMPLE PERMANENT TISSUES

The permanent tissue, whose cells are homogeneous i.e., having similar origin, structure and function, is called simple tissue. On the basis of the structure of constituent cells, simple tissues are of three types, parenchyma, collenchyma and sclerenchyma.

PARENCHYMA

Parenchyma (Gk. para = beside; en-chein = to pour; i.e., some semi liquid substance poured beside other solid tissues; Grew 1682) is the most basic type of differentiated tissue from which other types have evolved.

Characteristics

• The cells are living and may be rounded, oval, rectangular, star shaped but usually polygonal. Number of sides in a polygonal cell is usually 14 but in Elodea, it may be 17.
• Cells are loosely arranged with many intercellular spaces, either schizogenous or lysigenous (exception epidermis/epiblema, endodermis, pericycle and pith rays, where cells are compactly arranged).
• The cell wall is thin and is made up of cellulose, hemicellulose and pectin. Though the cell wall of xylem parenchyma is thick, that of epidermal cells is cutinised and that of endodermal cells are suberised.
• Cytoplasm is dense, granular with a central, large prominent nucleus.
• Vacuoles are many but small.
• Cytoplasm has all the cell organelles but instead of chloroplasts, there are leucoplasts.

MODIFICATIONS OF PARENCHYMA

To perform functions other than normal (storage of food), parenchyma gets modified into following types:

Chlorenchyma: Parenchyma cells having chloroplast; found in mesophyll of leaves; meant for photosynthesis.

Aerenchyma: Parenchyma with air filled spaces; found in hydrophytes, meant for buoyancy.

Prosenchyma: Parenchyma cells are much longer than their breadth and slightly thick walled to provide mechanical support. It can occur anywhere in the plant, more common in pericycle.

Stellate parenchyma: Parenchyma with star shaped cells; found in the leaf base of banana and Canna, meant for expansion of leaf base. Provides lightness as well as strength.

Idioblast: Specialized parenchyma whose cells secrete some secretions like mucilage, gums, resins, alkaloids, terpenes, etc. e.g., Citrus, Eucalyptus, etc.

Functions

• Collenchyma performs the following functions:
  • It is simple living mechanical tissue which provides mechanical support.
  • Being flexible in nature, it provides tensile strength to the plant body.
  • As the cells of collenchyma are living and often contain chloroplasts they also take part in photosynthesis.
  • It forms cork cambium in dicot stems during secondary growth.

Differences between parenchyma and collenchyma :

SCLERENCHYMA

Sclerenchyma (Gk. Scleros: hard; Mettenius 1805) is also a simple permanent tissue composed of thick walled, dead often lignified and hard cells. There is considerable variation in shape, size, origin and development of the cells. The mature sclerenchyma cells differ from parenchyma and collenchyma cells in the presence of lignified secondary walls and the absence of living protoplasts. Fibres and sclereids are the two common categories of sclerenchyma.

fibres

They are specialized sclerenchymatous cells that occur in different parts of the plant body in bands or in the form of uninterrupted hollow cylinder. They have great tensile strength, flexibility and elasticity which enable plant organs to withstand a variety of strains and tensions caused by the action of gravity, wind, etc.

Structure

• Fibres are long, narrow, thick and lignified sclerenchyma cells, usually with pointed or blunt ends. In transverse section, they appear rounded or polygonal in outline with a well defined lumen.
• Pits are few and simple, except fibre tracheids.
• Fibres are always found in sheets.
• Fibres are perhaps the longest cells among plant kingdom.

Types of Fibres

• On the basis of source and economic use, fibres are of following types:
  • Surface fibres: Fibres obtained from seed coat (testa) of cotton and mesocarp of coconut (coir of commerce) are surface fibres. Cotton fibres are of two types, lint and fuzz. The lint is longer and economically useful while the fuzz is shorter and is not useful.
  • Bast fibres: Fibres obtained from phloem and pericycle are bast or bass fibres. They are most exploited economically. e.g., Jute, Flax, Hemp.
  • Xylary or wood fibres: Fibres associated with xylem are called xylary fibres. Two types of xylary fibres are recognized, libriform fibres and fibre tracheids. Libriform fibres are longer, thick walled and with simple pits while fibre tracheids are shorter, less thickened usually with bordered pits.

Sclereids

Sclereids (Tschierch, 1885) are widely distributed and may occur almost anywhere in the plant body, either singly or in groups. When they occur singly, they are called sclerite or spicular cells or idioblastic cells.

Structure

• Sclereids, also known as stone cells or sclerotic cells, are isodiametric, spherical, oval, stellate, 'T' shaped or cylindrical in shape.
• They usually have thick and strongly lignified secondary walls with numerous simple pits. In mature sclereids, tannins and mucilage are often present as shrivelled remains of protoplasm.

Types of Sclereids

On the basis of the shape of cells and thickness of their walls, the following five types of sclereids have been recognized:

• Brachysclereids (stone cells): They are short and roughly isodiametric sclereids. They occur in the bark, cortex, pith and phloem of the stem of Cinnamomum and pulp of the fruits of Pyrus, Psidium guayava, etc. The grittiness in the pulp of these fruits is mainly due to the presence of these sclereids.
• Macrosclereids: They are columnar or rod like sclereids, present in seeds and fruits in the form of a layer. They occur commonly in leguminous seeds.
• Asterosclereids: These sclereids are star shaped. They occur in the leaves of Nymphaea and leaves and stems of Thea (tea) and Trochodendron.
• Osteosclereids: These sclereids are bone or barrel shaped with both ends more or less dilated. They are found in the leaves and seed coats of many dicotyledons, e.g., Hakea.
• Trichosclereids: They are hair like sclereids occasionally branched. They occur in the intercellular spaces in the leaves and stems of certain hydrophytes and in the aerial roots of Monstera.

Fig. Types of sclereids

Functions

The main functions of sclerenchyma are :

• It is mainly responsible for providing mechanical strength and rigidity to the plant.
• It saves the plant from various stresses and strains of environmental forces like strong winds, etc.
• It protects internal soft tissues.