Online Notes on Chapter = 6 ( TISSUE) Part 2

This is Biology Online Notes on Chapter = 6 ( TISSUE) Part 2 of 9th class of NCERT Book for preparation of examinations like CBSE Board, Biology Olympiad, NTSE, CSIR NET etc.

(b) Supporting Tissues : These are supportive in function and are of three types :

(i) Parenchyma : It is the fundamental tissue.

• Tissue first time evolved in bryophyte.

• Thin walled cells, oval or spherical in structure.

• Cell wall mainly composed of cellulose & pectin.

• Large central vacuole for food & water storage.

• Primary function is food storage.

• Some parenchyma involved in excretory substance storage are

   so called as idioblast, storing such as resin, tannin, gums & oils.

• In typical parenchyma chlorophyll is absent.

• Chloroplast containing parenchyma tissue are chlorenchyma

which perform photosynthesis e.g., mesophyll of leaves.

• In hydrophytic plants aerenchyma (a type of parenchyma

   containing air spaces) provides buoyancy.

• Parenchyma provides turgidity to cells.

(ii) Collenchyma : It is the living mechanical tissue.

• Elongated cells with thick corners.

• Localized cellulose & pectin thickening.

• Provides flexibility to plant parts & easy bending of various

   parts of plant.

• Present only in herbaceous dicot stem.

• Present at thin margin of leaves.

• Few chloroplasts may be present.

• Gives mechanical strength & elasticity to the growing stems.

(iii) Sclerenchyma : (Scleras – hard) Strengthening tissue.

• Composed of extremely thick walled cells with little or no

   protoplasm.

• Cells are dead & possess very thick lignified walls.

• Lignin is water-proof material.

• Intercellular spaces are absent.

Cells of sclerenchyma are of two types :

Sclereids :

• These are also called grit cells or stone cells.

• These are small cells, where lumen is so small due to higher             thickening of cell wall, as present in drup fruit (mango, coconut,      walnut) in legume seeds (Macrosclereid).

Fibers :

• They are very long, narrow, thick, lignified cells. Lumen is large       as compared to sclereids. Generally 1-3 mm long.

• In the thick walls of both the fibres and sclereids are present thin

   areas called as pits.

Sclrenchyma Fibres:

• These are used in the manufacture of ropes, mats & certain textile

   fibres.

• Jute and coir are obtained from the thick bundle of fibres.

(B) Complex Permanent Tissues:

• It consists of more than one type of cells which work together as a

   unit.

• It helps in transportation of organic materials, water & minerals.

• It is also known as conducting or vascular tissue.

• Xylem & phloem together form vascular bundles.

Xylem : Also known as wood and is a vascular and mechanical tissue.Thick walled cells are found in the form of tubular passages.

Xylem consists of four types of cells called as elements :

(i) Tracheids :

• They are elongated angular dead cells (primitive elements) mainly

    involved in conduction of water and minerals in gymnosperms.

(ii) Vessles : They are advance element (generally found in                     angiosperms).

• Vessels are cylindrical tube like structures placed one above the        other end to end which form a continuous channel for efficient        conduction of water.

(iii) Xylem parenchyma :

• They are small & thick walled parenchymatous cells subjected for

storage of starch (food).

(iv) Xylem sclerenchyma :

• Thy are non-living fibres with thick walls and narrow cavities           provide mechanical support.

• Except xylem parenchyma all other xylem elements are dead.

• The annual rings present in the trunk of a tree are xylem rings.

• By counting the number of annual rings, we can determine the         age of a tree.

Phloem : They also consist of both parenchymatous and schlerenc-

    -hymatous cells.

Phloem consists of four types of element :

(i) Sieve tubes :

• Sieve tubes are slender tube like structures made up of elongated,

   thin walled cells placed end to end.

• The end walls of sieve tube cells are perforated by numerous              pores, called as sieve plates.

• Nucleus of sieve cell degenerates at maturity. However, cytoplasm

   persists, because of protoplasmic continuation of sieve tube with

   companion cell through plasmodesmata.

• Sieve cells possess slime protein or protein which is concerned         with growth and repair of sieve cells.

(ii) Companion cells :

• Companion cells have dense cytoplasm and prominent nuclei.

   * Sieve cells & companion cells are so called sister cells because     they originate from single mother cell.

(iii) Phloem fibre :

• They give mechanical support to sieve tubes.

(iv) Phloem parenchyma :

• They store food and help in radial conduction of food.

(v) Leptome :

• Main part of phloem involved in conduction of food, which is           sieve tube.

• In xylem, only unidirectional movement is possible while in             phloem bidirectional movement can occur.

• In phloem, except phloem sclerenchyma all elements are living.

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