Plant tissues

ISC 12>STRUCTURE AND FUNCTION OF PLANTS>1. PLANT ANATOMY> Plant tissues

Scope of syllabus

Characteristics of meristematic tissue; classification of meristems based on origin, development, location and function;
A brief idea about differentiation, dedifferentiation and redifferentiation; tunica corpus theory;
Differences between shoot apex and root apex; brief understanding of histogen theory; quiescent centre;
Characteristics of permanent tissues; simple and complex tissues; types of vascular bundles to be taught with the help of diagrams;
Anatomical differences between dicot and monocot root, stem and leaf must be taught for better understanding. (Next )
Basic idea of how secondary growth takes place and formation of annual rings; structural and functional differences between heartwood and sapwood.(NEXT)

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Class Presentation

Plant body

Plant tissue

Plant tissues

What do you mean by tissues?
A group of cells with similar origin, structure and function is called tissue.

CLASSIFICATION OF PLANT TISSUES

Meristematic tissue:

Define meristematic tissues.
Meristematic tissue may be defined as a group of living cells which are located at specific location and divide continuously to add new cells to the plant body.

Characteristics of meristematic tissues.

Continuously dividing cells.
Cells are small and isodiametric.
The cell wall is thin and made up of cellulose.
Cytoplasm is abundant.
Nuclei are large.
Vacuoles are absent and if present, very few.
Lack inter-cellular spaces.
Metabolic activities are at high rate.
Inorganic inclusion absent.
Simple tissue

Classification of meristematic tissue

A) Based on Origin and development
(A) Promeristem:

The meristem where foundation of new organs or of their part is laid down is called promeristem.
They occupy very small area at the tips of stem and root.
It differentiates into primary meristem.

(b) Primary meristematic tissue

It originate from promeristem.
Remains meristematic from the embryonic condition throughout entire plant life at the growing apices of roots, stems, primordial of leaves.
Both apical meristems and intercalary meristems are primary meristems because they appear early in life of a plant and contribute to the formation of the primary plant body.

It develops from primary permanent tissue.
Fascicular vascular cambium, interfascicular cambium and cork-cambium.
Responsible for producing the secondary tissues.

B)Based on location in plant body

1. Apical meristem

Position: At the root tip and shoot tip.
Function: To increase in length of the axis.

2. Lateral meristem

Position: On the lateral side of stem and root.
Function: To increase in diameter of plant.

3. Intercalary meristem

Position:At the base of the leaf/internodes/ leaf.
Function:To increase the length of the plant or its organs.

Animations

C)Based on function

Protoderm- Developing into epidermis.
Procambium- Developing into vascular tissue.
Ground meristem- Developing into ground tissue and pith

A BRIEF IDEA ABOUT DIFFERENTIATION, DEDIFFERENTIATION AND REDIFFERENTIATION

Shoot apex

What do you mean by shoot apex....?

Shoot apex is the portion of shoot above the youngest primordium.
It is present at the plumular tip or at the end of leaf.
It is covered by young leaves and visible on removing them.
The apex is dome shaped.

Theories of shoot apex organisation:
1. Tunica corpus theory :(Schmidt-1924)

Observe the location of tunica and corpus regions

Draw the diagram

Tunica corpus theory was given for vegetative shoot apex.
According to this theory, there are two zones of tissues in the apical meristems.
The tunica (Tunic = cover) consisting of one or more layers of peripheral layers of cells, and the corpus (corpus = body) a mass of cells enclosed by the tunica.
The layers of tunica show anticlinal (perpendicular to periphery) divisions and bring about surface growth.
In the corpus, cell division is irregular and at various planes resulting in growth and volume of the mass.
Tunica gives rise to epidermis.
Corpus gives rise to Cortex/ground tissue.

2. Histogen Theory (Hanstein 1870)

According to this theory, shoot apex has three zones.
Apical meristem of stem composed of small mass of cells which are all alike and divide fast (meristematic).
These meristematic cells form promeristem, which differentiate into three zones : dermatogen, periblem and plerome.
Every zone consists of a group of initials called a histogen (tissue builder).

(i) Dermatogen (Outer layer): Gives rise to epidermis of stems.
(ii) Periblem: (Middle layer) Gives rise to cortex and endodermis.
(iii) Plerome: (Inner layer): gives rise to pericycle, pith and vascular tissue.

Shoot apex: Labeling exercise

Root apex

What do you mean by root apex....?

Root apex has simplest organisation as compared to stem.
Root apex is sub terminal in position because of terminal position of root cap.

Theories of root apex organisation:
1. Histogen Theory (Hanstein 1870)

According to this theory, root apex has three zones.
Apical meristem of root composed of small mass of cells which are all alike and divide fast (meristematic).
These meristematic cells form promeristem, which differentiate into three zones : dermatogen, periblem and plerome.
Every zone consists of a group of initials called a histogen (tissue builder).

(i) Dermatogen (Outer layer): Gives rise to epiblema of root .
(ii) Periblem: (Middle layer) Gives rise to cortex and endodermis.
(iii) Plerome: (Inner layer): gives rise to pericycle, pith and vascular tissue.

Calyptrogen:

The layer of meristematic cells at the apex of a root tip that continually cuts off new cells to its outer edge. These replace the cells of the root cap (calyptra) that are worn away as the root pushes through the soil.

Quiescent centre

Quiescent centre concept given by Clowes (1956) in maize.
According to this, there is an inactive centre in the root apex which is called quiescent centre (having low DNA, RNA, protein) and it acts as a reservoir of active initials.

Calyptra and calyptrogen

Calyptra: Root cap which protects the root tip.
Calyptrogen: Meristematic tissue which produce calyptra/root cap.

Permanent tissues

Permanent tissue may be defined as a group of living or dead cells formed by meristematic tissue and have lost their ability to divide and have permanently placed at fixed position in plant body.
I) Simple permanent tissues
II) Complex permanent tissues

I) Simple permanent tissues

TYPES OF SCLERENCHYMA

i) Fibres
Occurrence :In different plants parts.

A) Structure

Fibres are long, narrow, thick and lignified cells usually with pointed or blunt ends.
In transverse section, they appear rounded or polygonal with a well defined lumen.
Usually occur in bundles.

B) Functions

Provide tensile strength, flexibility and elasticity to plant or plant's parts.

C) Originate from meristematic cells.

ii) Sclereids ( Also known as stone cells or sclerotic cells) Occurrence : Most common in fruits and seeds

A) Structure

Cells are short, isodiametric, spherical, oval, t shaped or cylindrical in shape.
Usually have thick lignified secondary cell wall.
Occur single or in loose groups.

B) Functions

Protects the plant from stress and strains from environment.
Provide mechanical strength and rigidity.
Provide grittiness to the pulp of fruits.

C) Formed by secondary thickening of parenchyma cells.
.

II)Complex permanent tissue

A) Xylem :
i) Distribution in plant body:

Xylem & phloem in root

Xylem & phloem in stem

Xylem & phloem in leaf

ii) Components of xylem:

Components:

Tracheids
Vessels
Xylem parenchyma
Xylem fibre

Protoxylem and metaxylem

Difference between protoxylem and metaxylem

Primary and secondary xylem

Types of xylem : Endarch, exarch and mesarch
Based on relative position of protoxylem and metaxylem, the xylem is of the following three types.

Exarch: A condition in which the protoxylem lies towards the periphery and metaxylem towards the central axis. (Root)
Endarch: A condition in which protoxylem lies towards the centre and metaxylem towards the periphery.(Stem)
Mesarch: A condition in which metaxylem remain in the centre surrounded all around by the protoxylem.

B. Phloem
i) Distribution in plant body: (observe the figures given in xylem distriution)

ii) Components of phloem :

Components:

Sieve tube element
Companion cell
Phloem parenchyma
Phloem fibre

Sieve cell

In sieve cells the sieve area are not well differentiated.
The sieve cells have no companion cells.
These are elongated and quite long with tapering ends.
Found in pteridophytes and gymnosperms

Sieve tube

In sieve tubes the sieve area are confined to sieve plates.
The sieve tube always have companion cells.
They consist of vertical cells placed one above the other.
Found in angiosperms.

Primary phloem and secondary phloem

Differences between meristematic and permanent tissues

Types of vascular bundle

A. Based on cambium

2 Types

Open vascular bundle: Vascular bundle with cambium.
Closed vascular bundle : Vascular bundle without cambium.

DRAW THE DIAGRAM

B. Based on xylem and phloem arrangement

1. Radial vascular bundle:

In type of arrangement xylem and phloem form separate bundles alternating with each other i.e. they lie on different radii.
This arrangement is the characteristic of root.

DRAW THE DIAGRAM
Radial vascular bundle

2. Conjoint vascular bundle

Conjoint: When xylem and phloem are combine into one bundle.
a) Collateral: When xylem and phloem are situated at the same radius of vascular bundles.
b) Bicollateral: In this phloem and cambium lie on both sides of xylem. (Eg. Cucurbitaceae )

A) Conjoint collateral

DRAW THE DIAGRAM
Conjoint collateral

b) Conjoint bicollateral

DRAW THE DIAGRAM
Conjoint bicollateral

3.Concentric vascular bundle

When xylem and phloem arrange concentrically, i.e. one surrounds the other completely.
a) Amphicribal: When xylem lies in the centre and is completely surrounded by the phloem. (Ferns)
b) Amphivasal: When phloem lies in centre and is completely surrounded by the xylem. (Some monocot stem)

A) Amphicribal