Tissues

Tissues

Tissues

A group of cells that are similar in structure and/or work together to achieve a particular function forms a tissue.

Histology

The study of the tissue level of biological organization.

Plants are ______________ (movement)

Stationary

Why are plant tissues dead?

They can provide better mechanical strength than the live ones, and need less maintenance.

Most plant tissues are

Supportive

Types of Tissues

MERISTEMATIC TISSUE

• Apical

• Lateral

• Intercalary

PERMANENT TISSUE

• Simple

  • Parenchyma

  • Collenchyma

  • Sclerenchyma

• Complex

  • Xylem

    • Tracheids

    • Vessels (Trachae)

    • Xylem Parenchyma

    • Xylem Fibers

  • Phloem

    • Sieve tubes

    • Companion cells

    • Phloem parenchyma

    • Phloem fibres

Meristematic Tissues

• Continuously dividing

• Meristos: Greek for divided

• Growth of plants

• Cells in these tissues can divide to form new cells which differentiate to give rise to permanent tissue.

• Have spherical, oval, polygonal, or rectangular shapes, thin cellulose cell wall

• Have dense cytoplasm and a single large nucleus.

• No intercellular space.

Why do meristematic cells lack vacuoles?

• Constantly dividing

• Prominent nucleus and dense cytoplasm

• So, they don’t need to store food or waste

Types of Meristems

Meristematic tissues are of three types:

(i) Apical Meristem

• It is present at the apex of the growing tip of the stem and roots.

• Increases the length and region of active cell division.

(ii) Lateral Meristem (cambium)

• It is present beneath the bark.

• Have thin layers, cylindrical meristems, and are responsible for producing the secondary tissues.

• It is responsible for growth in the girth of the stem or root.

(iii) Intercalary Meristem

• It is present at the internodes or the base of the leaves and increases the length between the nodes.

• The meristem which occurs between mature tissues is known as intercalary meristem.

• They occur in grasses and regenerate parts removed by the grazing herbivores.

Onion Bulb Activity

Observation: Apical meristems were cut, so roots don’t grow

Primary Meristems

• Apical and Intercalary

• Appear in early life and contribute to formation of the plant body

Parenchyma

• Living

• Oval, elongated and loosely packed. Found throughout with basic packing of tissue

• Thin cell wall

• Mechanical Support

• Store food in vacuoles

Modifications

• Chlorenchyma

  • Parenchyma with chlorophyll for photosynthesis

• Aerenchyma

  • Aquatic plants: large air cavities for buoyancy

Collenchyma

• Living

• Oval, elongated, no intercellular spaces

• Below the epidermis

• Mechanical support

• Flexibility

• Thickened at he corners due to cellulose, hemicellulose and pectin depositions. This is called angular thickening.

Sclerenchyma

• Dead.

• Narrow and elongated.

• Composed of lignin which makes it hard.

• Found around vascular bundles, and veins of leaves in the hard covering of seeds and nuts.

• For example, Sclerenchyma tissues are found in a coconut husk.

  • They help to make parts of the plant hard and stiff.

  • Also provides mechanical strength.

Protective Tissues

• Epidermis

  • Covers the entire body

  • Protect from injury, germs and water loss

  • Continuous layer without intercellular spaces

  • Stomata are small openings on the epidermis of the leaf and soft prt of the stem to facilitate the gas exchange and transpiration

  • Each stoma is composed of guard cells

  • In desert plants, the epidermis and cutin (a waterproof waxy substance secreted by the epidermis) are thicker to reduce the loss of water due to transpiration.

  • Floating aquatic plants have stomata on the

    upper surface of leaves and remain in the

    open state. Submerged plants mostly do not

    have stomata and absorb nutrients and gases

    dissolved in water through the general

    surface.

• Cork

  • Dead cells with no intercellular spaces.

  • Outer layer of old tree trunks.

  • Have a chemical called suberin in their walls that makes them impervious to gases and water.

  • Protects plants from injuries, germs, and water loss.

  • Used to make several products like bottle stoppers and shuttle cork.

Difference between monocot and dicot stem

• monocot stem contains scattered vascular bundles across the stem

• dicot stem contains vascular bundles arranged in the form of one or two rings.

Xylem

• water and nutrients from roots to upper parts of the plant

• types:

  • Tracheids: long elongated cells with a tapered ending, dead, transport water through pits

  • Vessel: pipe-like, dead, have lignified thick cell walls, upper and lower portion of cell wall is absent, they’re a characteristic feature of angiosperms

  • Xylem Parenchyma: Living, store food and nutrient

  • Xylem Fibres: Dead, mechanical support

Phloem

• Tissue that transports food from leaves to other parts of the plant

• Phloem fibres: dead

• Types:

  • 1. Sieve tubes

    • are tubular, elongated, and have thin cell walls.

    • They have cytoplasm but no nucleus and other organelles.

    • These cells are responsible for the transportation of food and nutrients.

    2. Companion cells

    • have cytoplasm, nucleus, and other organelles.

    • They perform the tasks required for sieve tubes for living.

    3. Phloem parenchyma

    • store food and other substances like resins, latex, and mucilage.

    • Phloem parenchyma is absent in most of the monocotyledons.

    4. Phloem fibres

    • have thick cell walls and provide mechanical support to plants.

    • At maturity, these fibres lose their protoplasm and become dead. Phloem fibres of jute, flax, and hemp are used commercially.