PLANT TISSUES

A group of cells that are similar in structure and/or work together to achieve a particular function forms a tissue.
Most of the tissues in plants are supportive, which provides them with structural strength.
These tissues are dead since dead cells can provide mechanical strength as easily as live ones, and need less maintenance.
Plant Tissues are of two types Meristematic & Permanent tissues.

A. MERISTEMATIC TISSUE

These are simple living tissues having thin-walled compactly arranged immature cells which are capable of division and formation of new cells.

FEATURES OF MERISTEMATIC TISSUE

Thin primary cell wall (cellulosic).
Intercellular spaces are absent (compact tissue).
Generally, vacuoles are absent, dense cytoplasm & prominent nuclei are present.
Large numbers of cell organelles are present.
Active metabolic state, stored food is absent.
Actively dividing cells are present in growing regions of plants, for example, root & shoot tips.

I. CLASSIFICATION OF MERISTEMATIC TISSUES ON THE BASIS OF ORIGIN

1. PRIMARY MERISTEMATIC TISSUE(PROMERISTEM)

Derived directly from the meristems of the embryo.
They consist of cells derived from the primary meristem.
They add to the primary growth of plants.

2. SECONDARY MERISTEMATIC TISSUE

Formed by permanent tissues.
These are having cells derived from primary permanent tissue.
They usually add to the diameter of plants.

II. CLASSIFICATION OF MERISTEMATIC TISSUES ON THE BASIS OF LOCATION

1. APICAL MERISTEM

It is present at the growing tips of stems and roots.
Cell division in this tissue leads to the elongation of stem & root, thus it is involved in the primary growth of the plant.

2. INTERCALARY MERISTEM

It is present behind the apex.
It is the part of the apical meristem which is left behind
during the growth period.
These are present at the base of the leaf & internode
region.
These lead to the increase in the length of leaf
(Primary), for example: in grass stem, bamboo stem, mint
stem etc.

3. LATERAL MERISTEM

It is also called secondary meristem.
It occurs along the sides of the longitudinal axis of the plant.
It gives rise to the vascular tissues.
Causes growth in girth of stem & root.
They are responsible for secondary growth.

B. PERMANENT TISSUE

The permanent tissues are composed of those cells which have lost their capability to divide.
They have a definite shape, size and thickness. The permanent tissue may be dead or living.
The division & differentiation of the cells of meristematic tissues gives rise to permanent tissues.
In cell differentiation, developing tissue and organs change from simple to more complex forms to become specialized for specific functions.
The cells of permanent tissue lose the capacity to divide and attain a permanent shape, size and function.
Permanent tissues are classified into two types on the basis of Structure and Composition i.e. Simple Permanent Tissues and Complex Permanent Tissues.

SIMPLE PERMANENT TISSUE

These are made up of same type of cells which are similar structurally and functionally.
They include two types of tissue Protective tissues and Supporting Tissues.

PROTECTIVE TISSUE

These tissues are primarily protective in function.
They consist of Epidermis and Cork/Phellem.

EPIDERMIS

Epidermis forms one cell thick outermost layer of various body organs of plants such as leaves, flowers, stems and roots.
The epidermis is covered outside by a cuticle. The cuticle is a water-proof layer of a waxy substance called cutin which is secreted by the epidermal cells.
The cuticle is very thick in xerophytes.
Cells of epidermis of leaves are not continuous at some places due to the presence of small pores called as stomata.
Each stomata is guarded by a pair of bean-shaped cells called as guard cells. These are the only epidermal cells which possess chloroplasts, the rest being colourless.

FUNCTIONS OF EPIDERMIS

The main function of the epidermis is to protect the plant from desiccation and infection.
The cuticle of the epidermis cuts the rate of transpiration and evaporation of water and prevents wilting.
Stomata in the epidermis allow the gaseous exchange to occur during photosynthesis respiration.
Stomata also help in transpiration.

CORK OR PHELLEM

In older roots and stems, tissues at the periphery become cork cells or phellem cells.
Cork is made up of dead cells with thick walls and does not have any intercellular spaces.
The cell walls in cork deposit a waxy substance called suberin.
The cells of cork become impermeable to water and gases due to the deposition of suberin.
The cork cells are without any protoplasm but are filled with resins or tannins.

FUNCTIONS OF CORK

Cork is protective in function.
Cork cells prevent desiccation, infection and mechanical injury.
Imperviousness, lightness, toughness, compressibility and elasticity make the cork commercially valuable.
Cork is used for insulation, like a shock absorber in linoleum.
Cork is used in the making of a variety of sports goods such as cricket balls, table tennis, shuttlecocks, wooden paddles etc.

SUPPORTING TISSUE

These are supportive in function.
There are three types of Supporting tissues i.e.
(i) Parenchyma,
(ii) Collenchyma and
(iii) Sclerenchyma.

(i) PARENCHYMA

It is the fundamental tissue.
Tissue first time evolved in bryophyte.
Thin-walled cells, oval or spherical in structure.
The cell wall mainly composed of cellulose & pectin.
Large central vacuole for food & water storage.
The primary function is food storage.
Some parenchyma involved in excretory substance storage is so-called idioblast, storing such as resin, tannin, gums & oils.
In typical parenchyma chlorophyll is absent.
Chloroplast containing parenchyma tissue are chlorenchyma which performs photosynthesis such as 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 the plant.
Present only in herbaceous dicot stem.
Present at the 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 and Fibres.

SCLEREIDS

These are also called grit cells or stone cells.
These are small cells, where the lumen is so small due to higher thickening of the cell wall, as present in fruit (mango, coconut, walnut) in legume seeds (Macrosclereid).

FIBRES

They are very long, narrow, thick, lignified cells. Lumen is large as compared to sclereids. They are generally 1-3 mm long.
In the thick walls of both the fibres and sclereids are present thin areas called as pits.
Sclrenchyma Fibres are used in the manufacture of ropes, mats & certain textile fibres.
Jute and coir are obtained from the thick bundle of fibres.

DIFFERENCE BETWEEN PARENCHYMA, COLLENCHYMA AND SCELERENCHYMA

COMPLEX PERMANENT TISSUE

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

It is 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 Tracheids, Vessels, xylem parenchyma and xylem sclerenchyma.

(i)TRACHEIDS

They are elongated angular dead cells (primitive elements) mainly involved in conduction of water and minerals in gymnosperms.

(ii)VESSELS

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

They 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 schlerenchymatous cells.
Phloem consists of four types of element which are Sieve tubes, Companion cells, Phloem fibre and Phloem parenchyma.
In phloem, except phloem sclerenchyma, all elements are living.

(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.