Plant Tissue Notes

What is Tissue?

• A group of similar cells with a common origin performing specific functions.
• Term introduced by Bichat.
• Study of tissues is Histology.
• Bichat is considered the "Father of Histology".

Meristematic Tissues

• Cells with the ability to divide.
• Types:
  • Apical (top)
  • Lateral
  • Intercalary (intermediate)

Permanent Tissues

• Derived from meristematic tissues, lost ability to divide.
• Mature form.
• Classified into simple and complex.

Similarities Between MT & PT

• Both are tissue systems in plants.
• Made of cells performing specific functions.
• Homogenous in nature with single cell types.

Differences Between Meristematic and Permanent Tissues

• Cell Size & Shape:
  • MT: Small, isodiametric, large lumen.
  • PT: Large, different shapes.
• Living/Non-living:
  • MT: All living.
  • PT: Both living and non-living.
• Cytoplasm:
  • MT: Dense.
  • PT: Death cells of sclerenchyma—no cytoplasm
• Cell Wall:
  • MT: Cellulose, no secondary thickening.
  • PT: Thick or thin, lignified, secondary thickening.
• Intercellular Space:
  • MT: Absent.
  • PT: Loosely or compactly packed.
• Vacuoles:
  • MT: No vacuoles.
  • PT: May or may not have.
• Location:
  • MT: Certain parts of plant.
  • PT: Throughout plant.
• Nucleus:
  • MT: Prominent.
  • PT: May or may not have.
• Cell Type:
  • MT: Simple tissue.
  • PT: Simple or complex.
• Ability to Divide:
  • MT: Present, undifferentiated cells.
  • PT: Differentiated, no ability to divide.
• Examples:
  • MT: Shoot and root apex.
  • PT: Cortex of stem/roots, mesophyll.

Characteristics of Meristem

• Living and thin-walled.
• Rounded or polygonal.
• Compactly arranged.
• Thin and elastic cell wall.
• Few small vacuoles.
• Dense cytoplasm, distinct large nucleus.
• Absent plastids but present proplastids.
• Abundant ribosomes, simple ER.
• Simple abundant mitochondria.
• Absent crystals.
• High biosynthetic activity.
• Found at root and shoot apices.

Classification of Meristematic Tissues Based on Origin

• Promeristem:
  • Earliest, youngest.
  • Originates from the embryo.
  • Gives rise to primary meristem.
  • Tips of root and shoot.
• Primary Meristem:
  • From promeristem.
  • Divides throughout life.
  • Gives rise to primary tissue system.
  • Examples: Apical/Terminal, Intercalary, Intrafasicular cambium.
• Secondary Meristem:
  • Develops in primary permanent tissues.
  • Gives rise to secondary permanent tissues.
  • Helps add girth.
  • Examples: Cork cambium, Intrafascicular cambium, Vascular cambium, Wound cambium.

Classification Based on Location

• Apical
• Intercalary
• Lateral

Apical Meristems

• Tips of stem, root, branches.
• Give rise to primary permanent tissues.
• Constitutes primary body.
• Shoot apical meristem is terminal.
• Root apical meristem is sub-terminal.

Intercalary Meristems

• Base of leaves and nodes.
• Longitudinal growth.
• Erects fallen stems.
• Short-lived.

Lateral Meristems

• Sides of stems and roots of dicots and gymnosperms.
• Rectangular cells, divide tangentially.
• Vascular bundles, cork cambium.
• Increases girth/thickness.

Note

• Increase in girth is secondary growth.
• Apical meristem increases height.

Structure and Organization of Apical Meristems

• Shoot Apical Meristem (SAM):
  • From meristem in plumule of embryo.
  • Tip of shoot and branches, axils of leaves.
  • Conical or dome-shaped, protected by leaves.
  • Elongation of shoot.
• Reproductive Shoot Apex (Floral bud)-RSAM
  • Vegetative apices transform during reproductive phase.
  • Apex stops leaf primordia production.
• Root Apical Meristem (RAM)
  • Tip of main roots and branches.
  • Sub-terminal due to root cap.

Three meristematic regions of root apical meristem:

• Formed from:
  * Protoderm—it forms epiblema and root cap (calyptra).
  * Procambium—vascular tissue
  * Ground meristerm—pith, endodermis and cortex.

Permanent Tissues

• Mature, differentiated cells.
• From meristematic tissues.
• Fixed position.
• Definite shape, size, and function.
• Intercellular spaces, large vacuoles.
• Lost power of division.
• Living or dead.
• Support, protection, photosynthesis, conduction.
• Primary - derived from apical and intercalary meristem
• Secondary - derived from lateral meristem
• Simple and Compound types.

Simple Permanent Tissues

• Homogenous tissue
• Similar structure, function, origin, and form.
• Types: Parenchyma, Collenchyma, Sclerenchyma.

Parenchyma

• Living, isodiametric, thin-walled cells.
• Oval, spherical, or polygonal.
• Simple and primitive.
• Large vacuole.
• Intercellular spaces.
• Fundamental tissue.
• Non-woody parts.
• Packing tissue, mechanical support, storage.

Types of Parenchyma

• Epidermal
  • Outermost covering.
• Chlorenchyma
  • Contain chloroplasts.
• Aerenchyma
  • Large air-filled intercellular spaces.
  • Buoyancy in aquatic plants.
• Prosenchyma
  • Elongated and thick-walled.
  • Mechanical strength.
• Vascular parenchyma
  • In xylem and phloem.
• Idioblasts
  • Store tannins, oils, crystals.
• Succulent
  • Store water.
• Storage
  • Store water, starch, proteins.

Functions of Parenchyma

• Storage
• Transport
• Photosynthesis
• Gas Exchange
• Protection
• Buoyancy

Collenchyma

• One type of cell, different shapes.
• Circular, oval, or polyhedral.
• Living, vacuolated protoplasm.
• May or may not be present intercellular spaces.
• Uneven thickenings on cell wall.
• Cellulose, hemicellulose, and pectin.
• Absent lignin.
• Stem, petiole, leaves of herbaceous dicots.
• Few chloroplasts or none.
• Absent in roots and monocots.
• Flexibility and mechanical support.

Angulate, Lamellate and Lacunate

• Thickness can be primarily deposited at the corners or angles where cells meet.
  • E.g. Hypodermis of Datura and tomato stem
• Thinkening are deposited more in the tangential walls
  • E.g. Hypodermis of Sunflower (Helianthus) and Raphanus (radish) stem.
• Thickening is primarily deposited around the intercellular spaces.
• E.g. Hypodermis of Curcurbita stem and IPomoea

Sclerenchyma

• Thick-walled, lignified dead cells.
• Various shapes and sizes.
• Mechanical support.
• Two types: fibres and sclereids.

Sclerenchyma Fibres

• Elongated with tapering ends.
• Dead.
• Cellulose or lignin.
• Reduced lumen.
• Hypodermis, pericycle, secondary xylem and phloem.

Types of Sclerenchyma Fibres

• Bast fibres: Found in the pericycle and phloem.
• Wood fibres: Found in the secondary xylem tissue or wood.

Stone Cells or Sclereids

• Short, isodiametric cells.
• Thick-walled, lignified, dead.
• Narrow lumen.
• Walls with pits.
• Mechanical support and hard texture.
• Cortex and phloem.
• Grittiness to fruits.
• Types of Sclereids.

Functions of Sclerenchyma

• Protects from stress.
• Mechanical strength and rigidity.
• Grittiness to fruits.
• Dispersal by wind.

Complex Permanent Tissues

• More than one cell type, common origin.
• Special functions.
• Two types: Xylem and Phloem.

Xylem

• Conducts water and minerals and provides mechanical support.
• Stems, roots, leaves, flowers, and fruits.
• Living and non-living cells.
• Four elements: Tracheids, Vessels, Xylem parenchyma, Xylem fibres.

Tracheids

• Elongated dead cells, large central cavity.
• Tube-like, tapering ends.
• Thick, lignified walls with pits.
• Ferns and gymnosperms.
• Associated with vessels in angiosperms.
• Conduction of water and minerals and mechanical support.

Types of Tracheids Based on Secondary Thickenings

• Annular
• Spiral
• Reticulate
• Scalariform
• Pitted

Vessels (Tracheae)

• Similar to tracheids but cylindrical cells arranged end-to-end.
• End walls dissolved.
• Dead cells, absent in pteridophytes and gymnosperms.
• Walls thickened and lignified.

Xylem (Wood) Parenchyma

• Living parenchymatous cells.
• Thin cell wall.
• Storage of reserve food, assists in conduction.

Xylem (Wood) Fibres

• Dead sclerenchymatous fibres.
• Thick lignified walls, narrow lumen.
• Provide mechanical strength.

Classification of Xylem Based on the Type of Growth Associated

• Primary xylem
• Secondary xylem

Primary Xylem

• Associated with primary growth.
  • Develops from procambium.
    • Made up of tracheids, vessels, fibres and parenchyma cells.
  • Not differentiated in axial and ray parenchyma.
  • Present only axial parenchyma.

Two Types of Primary Xylem

• Protoxylem
• Metaxylem

Comparison of Protoxylem and Metaxylem

• Protoxylem:
  • Early formed.
  • Narrow vessels.
  • Short period.
  • Tracheids, vessels, parenchyma.
• Metaxylem:
  • Later formed.
  • Large vessels.
  • Long periods.
  • Tracheids, vessels, parenchyma and fibres.

Secondary Xylem

• Associated with secondary growth.
  • Develops from vascular cambium.
    • Made up of tracheids, vessels, various fibres (i.e. Tracheids fibres and libriform fibres) and xylem parenchyma.
    • Differentiated in axial and ray parenchyma (present both).
    • Present pitted elements.

Three types of xylem based on position of protoxylem and metaxylem

• Endarch
• Exarch
• Mesarch
  • Metaxylem in centre surrounded completely by protoxylem. E.g. Found in leaves of ferns and cycads.

Function of Xylem

• Conduction of water, minerals, and nutrients.
• Mechanical support.

Phloem

• Mainly conduct food
• Composed of sieve elements, comanion cells, phloem parenchyma and phloem (bast) fibres.

Seive tube elements

• Living, slender and elongated cells placed end-to-end.
  • Present large cavities (spaces) with thin cell walls (made up of cellulose).
  • In the transverse walls present a number of pores called the sieve pits or sieve pores.
  • Absent nucleus in seive tubes.
  • Function—helps in conduction of food (amino acids & carbohydrates) from leaves to other regions.

Companion cells

• Lying parallel to sieve tube elements.
  • Present prominent nucleus and dense cytoplasm.
  • Nucleus of companion cells regulates activities of sieve elements.
  • Found only in angiosperm, absent in pteridophytes and gymnosperms.

Phloem

• Associated with primary growth of plant body.

  • Not differentiated in axil and ray system
  • Made up of sieve elements narrow and inconspicuous and seive elements are longer and wider