Plant tissues-Living tissues

Plant tissues

Let's do the activity given below to study about plant tissues.

Activity

Materials required :- Thin peel of lower epidermis of betel leaf, thin section of a potato tuber, thin cross section of a stem of a plant like Balsam

Method :- 

• Prepare temporary slides using above plant materials. 
• Observe them under microscope.
• Try to identify tissues formed by cells with the help of your teacher.

View of different plant tissues under optical microscope Activity

You may have observed that plant tissues are of different forms. Animal tissues are also of different forms. It is observable that different cell types are present in living beings and similar cells are arranged together.
A group of cells with a common origin that has been modified to perform a specific function in the body is known as a tissue.

Classification of plant tissues
Let's do the following activity to study further about how tissues are oraganized in plant organs.

Activity

Materials required :- Prop root of Banion/ Stilt root of Pandanus/ Stilt root of Rampe

Method :-

• Observe the external view of the growing tips of above roots 
• Use a hand lens to observe them

External view of a root 

We can identify the nature of growing root from the above observation. Growing part is soft and light coloured. Mature part is rough and dark coloured. This is because of the nature of tissues.

Below figure shows the microscopic view of longitudinal section of such root.

Microscopic view of longitudinal section of root apex

It is observed that region A-A' has cells with the ability of cell division. Region B-B' contain different cell types that are different in nature from cells in region A-A'.

Plant tissues can be categorised using different criteria. Plant tissues can be divided into two groups according to the ability of cell division.

• Meristematic tissues
• Permanent tissues

Permanent tissues can be found in region B-B' and Meristematic tissues in region A-A' as shown above figure.

Meristematic tissues

The tissues with cells that divide actively by mitosis to produce new cells are called meristematic tissues. These cells are not differentiated. The growth of plants takes place due to activity of meristematic tissues.

Features of meristematic tissues

• This tissue consists of small sized living cells
• No inter cellular spaces or intercellular spaces are not prominent
• There is a distint nucleus in each cell
• Absence of large central vacuole but small vacuoles may be present
• Large number of mitochondria are present

Meristematic tissues are present in specific locations of the plant. They are of three types.

Locations of meristematic tissues

Apical meristems

Apical meristems are found in shoot apex, root apex and axillary buds. Plant increases its height due to the activity of this tissue.

Intercalary meristems

Intercalary meristems are found at nodes. The length of internode increases due to the activity of the above tissue. They are found in plants of grass family

Lateral meristems

Lateral meristems are present laterally in the stem and roots of plant. They are found parallel to the longitudinal axis of plant. The diameter of the plant increases due to the activity of this tissue. Cambium tissue found in dicots is a lateral meristematic tissue.

Permanent tissues
Do the following activity to identify different types of tissues of plant stem.

Activity

Materials required :- Pumpkin / Tridex plant stem, A glass slide, A microscope
Method :-

• Observe the cross section of above plant stem using a microscope
• Identify the different tissue types present in them

Microscopic view of cross section of dicot stem

A specialized tissue that lost its ability to divide and perform a particular function is known as a permenant tissue.
According to the nature of the permanent tissues, it can be grouped into two.

• Simple Permanent tissues - One type of cells collected together
• Complex Permanent tissues - Different types of cells collected together

?Simple permanent tissues
The tissue is composed of similar cells. According to the shape of cell and the nature of cell wall, three types of simple permanent tissues as parenchyma, sclerenchyma, and collenchyma can be identified in plants.

Simple permanent tissue types

Parenchyma
The tissue that forms the soft parts of the plant body is the parenchyma tissue. This is the most abundant tissue found in the plant.

Features of Parenchyma tissue

Parenchyma tissues

• Parenchyma tissue consists of living cells.
• Cells are isodiametric (spherical) with a large central vacuole.
• Nucleus is present peripherally in the cytoplasm
• The cell wall is thin and made up of cellulose.
• Inter cellular spaces are present.

Locations of parenchyma tissues

• Cortex and pith of plant stem 
• Pith and cortex of roots
• Fleshy parts of fruits 
• Seeds (endosperm)
• Leaves (mesophylls)

Functions of parenchyma

• Photosynthesis        - Palisade and spongy mesophylls in plant leaves contain chlorophyll                                             within chloroplasts. Photosynthesis takes place within these chloroplasts.
• Food storage           - Food is stored in some parenchyma tissues and they are called as storage                                     tissues. E.g. : Potato tuber, Carrot and Sweet potato roots, Papaw and                                           Banana fruits.
• Storage of water     - Specially xerophytic plants store water in parenchyma tissue E.g. : Aloe                                       leaves, Bryophyllum leaves, Cactus cladode
• Providing support   - Herbaceous plants like Balsam absorb water into vacuoles of the                                                 parenchyma cells. Thereby cells become turgid and provide mechanical                                       support to the plant.

Collenchyma

Collenchyma tissue provides mechanical strength and support to the plant body. They are modified parenchyma cells.

Collenchyma tissue

Features of collenchyma tissue

• Collenchyma tissue consists of living cells
• Cells possess a cytoplasm, nucleus and central vacuole
• Generally cells are elongated and polygonal in cross section
• The corners of the cell walls are thickened.
• Intercellular spaces may present or may not present

Locations of collenchyma
The collenchyma forms a cylindrical tissue inner to the epidermis of herbaceous stems. They are found in the veins of dicot leaves.
Functions of collenchyma
1. Support             - Collenchyma provides mechanical support to dicot plant stem, before the                                               formation of wood. (Provides mechanical support to herbs)
                                This tissue provides support to the plant leaves by the collenchyma in veins.
2. Photosynthesis - Chloroplasts are found in the collenchyma of immature dicot stems.                                                       Photosynthesis is carried out by those cells.

Sclerenchyma

Sclerenchyma tissue

Sclerenchyma tissue helps in providing mechanical strength and support to the plant body. This tissue has two types of cells as sclereids and sclerenchyma fibres.

Features of sclerenchyma tissue

• Sclerenchyma tissue consists of dead cells
• Lignin is deposited on the cellulose cell wall
• Cells are tightly packed. Therefore, no intercellular spaces
• Cell wall is evenly thickened and forms a central lumen

Locations of sclerenchyma tissue

Fibres present in xylem are called as xylem fibres and in phloem as phloem fibres. Other than above, coconut fibres, agave fibres and cotton wool are made up of fibres (sclerenchyma)

Selereids are found in endocarp of cocount, camphor and mango fruits, the pericarp of guava fruit and in pear fruit and seed coat of coffee and dates.

Functions of sclerenchyma

• Provide support to the plant body

Complex permanent tissues

Different types of cells together form a complex permanent tissue. Two complex permanent tissues as xylem and phloem can be identified in plants.

Complex permanent tissues

Xylem and phloem are found in vascular systems of root, stem and leaves of the plant.

Xylem tissue

Xylem tissue

This tissue is composed of four different types of cells.

• Xylem vessel element
• Tracheids
• Fibres
• Parenchyma cells

Xylem vessel elements are cylindrical elongated cells. Xylem vessel element stack on top of the others and the cross walls are dissolved to form a continuous xylem vessel. This tubular structure helps in transportation of water in plants. Tracheids are elongated, spindle shaped cells. They also help in transportation of water. Xylem fibers are narrower and shorter than tracheids. Xylem vessels, tracheids and xylem fibers become dead due to lignification of cell walls. They provide support to the xylem tissue. Xylem parenchyma are living cells with a thin cell wall. They involve in food storage.

Functions of xylem

• Transportation of water and minerals to the plant body which are absorbed by plant roots.
• Providing of mechanical support to plant body.

Pholem tissue

Longitudinal section of the phloem tissue

Cross section of the phloem tissue

Phloem tissue is composed of four different types of cells.

• Sieve tube elements
• Companion cells
• Phloem fibres
• Phloem parenchyma

The sieve tube elements fuse end to end and the cross walls are incompletely dissolved to form a sieve tube. The cross walls in these sieve tubes are called sieve plates. Sieve tubes transport food (mainly sucrose) throughout the plant, as a solution.

Companion cells are elongated cells associated with sieve tube elements. The nucleus of the companion cell controls the activities of the sieve tube elements (sieve tube elements lack a nucleus) Seive tube elements, companion cells and phloem parenchyma are living cells whereas phloem fibres are dead and found scattered in phloem tissue.

Functions of phloem tissue

The food synthesized in the leaves are transported throughout the plant body by this tissue (Translocaton).

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