Tissues

Tissue is a cellular organizational level intermediate between cells and a complete organism. Hence, a tissue is an ensemble of cells, not necessarily identical, but from the same origin, that together carry out a specific function.

The study of tissue is known as histology or, in connection with disease, histopathology.

Tissues in PlantsPlant Tissues

Meristimatic Tissues (These cells continuously divide throughout the life of the plant)

Apical Meriste

m

Lateral Meristem

Intercalary

Meristem

Permanent Tissues(Cells with speciallised structure and function that have lost their ability to

divide further)

Simple Permanent Tissue

Parenchyma

Collenchyma

Sclerenchyma

Complex Permanent Tissue

Xylem

Phloem

Tissues in PlantsMeristematic tissue: Cells of this tissue continue to divide throughout the life of the plant. Some of these cells lose their ability to divide and become part of other tissues.

Name of the tissue

Location Function

Apical Meristem Present at the growing tip of stem and root

Increase in length of stem and root

Lateral Meristem (also called cambium)

Found on the lateral sides of roots and stem

Increase in girth of stem and root

Intercalary Meristem

Present at the base of leaves or internodes

Growth of leaves and branches

Tissues in Plants

Shoot

Root

Tissues in Plants

A longitudinal section through a growing shoot tip showing apical meristematic tissue. Note that the cells are small, have dense

cytoplasm, and are very tightly packed.

Tissues in Plants

High power view of a longitudinal section of the Coleus apical

meristem. The apical meristem is a dome-shaped mass of dividing cells

at the tip of the shoot. The apical meristem will produce the three primary meristems: protoderm,

procambium, and ground meristem. These three meristems in turn will produce new cells that

will differentiate into the epidermis, primary vascular

tissues, and ground tissues (pith and cortex).

Tissues in Plants

A longitudinal section through a root tip. The meristematic tissue is located just above the root cap. This too is apical meristem; division of these cells followed by cell elongation results in the root growing

in length.

Root cap

Apical Meristem

Zone ofCell

elongation

Tissues in Plants

It is a cross section of a dicot stem. Focus on the two large vascular bundles in the center of the slide. The xylem tissue is stained red. Just above the xylem is a layer of meristematic tissue, the vascular cambium.The phloem tissue is found outside of the vascular cambium.

Tissues in Plants

This is a high-power view of a cross-section showing a lateral meristem, the vascular cambium, in the same plant shown in previous slide. Again, the xylem tissue is stained red, and the large cells on the top

of the slide are phloem. The green brick-like cells between the xylem and phloem is the area in which the vascular cambium is located. The new cells produced by the cambium are initially like those of the cambium itself, but, as they grow and mature, their characteristics slowly change as they differentiate

into other tissues. The vascular cambium is a single layer of cells within this brick like region; it is responsible for the growth in diameter of a stem. The tissues produced by the vascular cambium are

secondary tissues.

Tissues in PlantsPermanent tissue: Cells of this tissue have lost their ability to divide and they have a specialized structure to perform specific functions.

Based on the type of cells present in the tissue, the Permanent tissue is divided into two categories:Simple Permanent Tissue and Complex Permanent Tissue.

While the simple permanent tissue consist of only one type of cells (eg. Parenchyma), the complex permanent tissue consists of more than one type of cells (eg. Xylem and phloem)

Tissues in PlantsS i m p l e P e r m a n e n t T i s s u e s

ParenchymaStructure:It is the fundamental tissue composed of thin walled, living cells whose cell wall is composed of cellulose. Small intercellular spaces are present between the cells.

Location and function: It occurs in all soft parts of plants and is meant for storage of food and to provide turgidity to softer parts of plants. Parenchyma tissue in stem and roots store nutrients and water.

Types of parenchyma:

i) Chlorenchyma :Certain parenchymatous tissue contain chloroplast and synthesize food by the process of photosynthesis.

ii) Aerenchyma: In aquatic plants parenchymatous cells have air cavities between them to store air, such a tissue is called Aerenchyma. It provides buoyancy to the aquatic plants so that they can float in water.

Tissues in Plants

Aerenchyma tissue

Tissues in Plants

Collenchyma

Structure: This tissue is composed of somewhat elongated cells with cell walls that are irregularly thickened at corners due to deposition of cellulose or pectin. They may be oval, circular or polygonal. Very little intercellular spaces are present.

Location: It occurs below the epidermis of stem and petiole (stalk of the leaf) and around veins.

Function: This tissue provides mechanical support and flexibility and in some cases it may possess chloroplasts to perform Photosynthesis. The stem and leaves are able to bend easily and then come back to their original position due to the presence of collenchyma.

Tissues in Plants Collenchyma in Transverse Section Showing Wall Thickenings1. Cell Wall2. Wall Thickenings3. Protoplasm4. Vacuole

Tissues in Plants

Sclerenchyma

Structure: It is a tissue of dead and thick walled cells, having no intercellular spaces. The thickenings are of cellulose or lignin or both. Several unlignified areas called pits often develop on walls.

Location: This tissue is usually found in the hard and stiff parts of the plant like seed coat, husk of coconut, in the stem around vascular bundles, veins of leaves and hard covering of fruits and nuts.

Function: It is the chief mechanical tissue in plants and is able to bear push, pull, strain and shearing forces. It provides strength to plant parts and also protects the delicate parts of the plants.

They are of two types: fibres and sclereids.

Tissues in Plants

Sclerenchyma

Tissues in Plants

Tissues in Plants

Tissues in Plants

Epidermis and BarkThe protective tissues

The epidermis usually consists of a single-layered group of cells that covers plants leaves, flowers, roots and stems. It forms a boundary between the plant and the external world.

Bark is formed from the meristem that appears later in the life cycle of a plant. Woody stems and some other stem structures produce a secondary covering called the secondary meristem or periderm or cork cambium that replaces the epidermis as the protective covering.The periderm replaces the epidermis, and acts as a protective covering like the epidermis. Cells produced on the outside by periderm form the cork. Cells of have suberin in their walls to protect the stem from drying and pathogen attack. Older cork cells are dead, as is the case with woody stems. As the stem grows, the cork cambium produces new layers of cork which are impermeable to gases and water.

Tissues in PlantsEpidermis

Tissues in PlantsEpidermis

Tissues in Plants

A high-power view of one glandular hair. Secretory hairs may provide a chemical defense against insects.

Tissues in Plants BARK

Tissues in Plants

Another type of surface tissue, the outer bark or periderm (stained red in this slide). Periderm is found on the surface of woody plants; it includes the cork cells on the

surface of older woody stems. The periderm replaces the epidermis in plants that have secondary growth. The cork cells are dead; it is their waterproofed cell walls that function as the protective outer covering of plants. Meristematic cells within the

periderm (cork cambium, the other lateral meristem) produce the cork cells.

BARK

Tissues in PlantsC o m p l e x P e r m a n e n t T i s s u e s

Xylem and Phloem

Tissues in Plants

It is a complex permanent tissue, which is specialized for the conduction of water and mineral substances in the plant body. Xylem is a heterogenous tissue made up of four different types of cellular elements.

They are:

•Xylem tracheids

•Xylem tracheae or vessels

•Xylem fibers and

•Xylem parenchyma

Xylem

Tissues in PlantsPhloem:

Phloem is a complex permanent tissue, which is specialized for the conduction of food and other organic substances. Phloem is also a heterogenous tissue, made up of four different types of cellular elements, namely,

•Sieve tubes

•Companion cells

•Phloem parenchyma and

•Phloem fibres

Tissues in Plants