Anatomy of Flowering Plants

The Tissues

• A tissue is a group of cells with a common origin and function.
• Plants have different kinds of tissues.
• Tissues are classified into meristematic and permanent tissues based on their ability to divide.

Meristematic Tissues

• Meristems: Regions of active cell division in plants or simply say “Active growth centers”.
• Types of Meristems:
  • Apical Meristems:
    • Found at the tips of roots and shoots.
    • Produce primary tissues.
    • Root apical meristem is located at the root tip.
    • Shoot apical meristem is present at the tip of the stem.
    • Axillary buds arise from cells left behind and form branches or flowers.
      • These buds remain dormant or grow into lateral branches or floral shoots.
  • Intercalary Meristems:
    • Found between mature tissues.
    • Occur mainly in grasses.
    • Help regenerate parts removed by grazing.
    • “They allow regrowth of leaves and stems after cutting or grazing.“
  • Lateral Meristems:
    • Found in mature regions of roots and shoots.
    • Produce secondary tissues.
    • Examples: vascular cambium, cork cambium.
    • “These meristems are responsible for increase in thickness of stem and root.”

Based on Time of Appearance

• Primary Meristems: Appear early in the plant’s life.
  • “They form the primary plant body.”
• Secondary Meristems: Appear later and produce woody tissues.
  • “They arise from permanent tissues by regaining the power of division.”
• Cells from meristems become specialized and form permanent tissues.

Some meristems gradually become specialised and form permanent tissues through differentiation.

“This process results in formation of tissues with specific structure and function.“

Permanent Tissues

• These cells do not divide further.
• “These cells lose the power of division after attaining a definite shape, size, and function.”
• 2 Types: simple tissues and complex tissues.

A. Simple Tissues

• Made of one type of cell.
• “All cells of a simple tissue are structurally and functionally similar.”
• Types of Simple Tissues:
  1. Parenchyma:
     • Major component of plant organs.
     • Cells are isodiametric (spherical, oval, round, polygonal, or elongated).
     • Thin cell walls, made of cellulose.
     • Cells may be closely packed or may have intercellular spaces.
     • Functions:
       • Photosynthesis,
       • Storage,
       • Secretion
     • “Parenchyma present in leaf mesophyll containing chloroplasts is called chlorenchyma and performs photosynthesis.”
     • “In aquatic plants, parenchyma with large intercellular spaces is called aerenchyma and helps in buoyancy.”
  2. Collenchyma:
     • Found just below the epidermis in dicots.
     • Cells are thickened at corners due to cellulose, hemicellulose, and pectin deposition.
     • Cells may contain chloroplasts for food assimilation.
     • No intercellular spaces.
     • Functions:
       • Provides mechanical support to growing parts such as young stems and leaf petioles.
     • “Collenchyma occurs as continuous layers or in patches beneath the epidermis.“
  3. Sclerenchyma:
     • Cells are long, narrow, and have thick, lignified walls with pits.
     • Cells usually dead and lack protoplasts.
     • Provides mechanical support to plant organs.
     • Types: Fibres & Sclereids
       1. Fibres:
          • Thick-walled, elongated, tapering cells with pointed ends.
          • “They possess a narrow lumen and numerous pits.”
          • Usually found in groups in various plant parts.
       2. Sclereids:
          • Spherical, oval, or cylindrical cells with very narrow cavities (lumen).
          • Found in
            • Fruit walls of nuts
            • Pulp of fruits like guava, pear, sapota
            • Endocarp of drupe fruits
            • Seed coats of legumes
            • Tea leaves
          • Provides mechanical support to organs.

“Coir of coconut is an example of sclerenchyma fibres.”

B. Complex Tissues

• Complex Tissues: Made of more than one type of cells working together as a unit.
• “Such tissues are heterogeneous and are also called vascular or conducting tissues.”
• Types: Xylem and Phloem.

1. Xylem

• Function: Conducts water and minerals from roots to stem and leaves, provides mechanical strength.
• Components:
  1. Tracheids:
     • Elongated, tube-like cells with thick, lignified walls and tapering ends.
     • Dead cells without protoplasm.
     • “Wall thickenings may be annular, spiral, reticulate, scalariform, or pitted.”
     • “They are the primitive conducting elements present mainly in pteridophytes and gymnosperms.”
  2. Vessels:
     • Long cylindrical tubes formed by a series of vessel elements.
     • Walls are lignified; protoplasm absent.
     • Lack nucleus or any other organelles at maturity.
     • “Vessels are characteristic of angiosperms and have perforated end walls for efficient water conduction.”
     • “Exceptions: Some gymnosperms like Ephedra and Gnetum has vessels .”
  3. Xylem Fibres:
     • Thick-walled sclerenchymatous cells with narrow or obliterated lumen.
     • May be septate or aseptate.
     • Function: Mechanical strength.
  4. Xylem Parenchyma:
     • Living, thin-walled cells made of cellulose.
     • Store food such as starch and fats.
     • “They also help in radial conduction of water through ray parenchyma.”
• Types of Primary Xylem:
  1. Protoxylem:
     • First formed xylem.
     • Lies towards center in stems (endarch),
     • Lies towards periphery in roots (exarch).
  2. Metaxylem:
     • Later formed, lies towards periphery in stems and center in roots.
     • “Reticulate and pitted thickenings are common in metaxylem.”

2. Phloem

• Function: Transports food materials from leaves to other parts of the plant.
• Components:
  1. Sieve Tube Elements:
     • Long, tube-like living cells arranged longitudinally.
     • End walls are perforated to form sieve plates.
     • Lack nucleus at maturity.
     • “They possess peripheral cytoplasm and a large vacuole.”
     • “Their functions are controlled by the companion cells.”
  2. Companion Cells:
     • Specialized parenchyma cells closely associated with sieve tubes.
     • “They help in maintaining pressure gradient in sieve tubes.”
     • “Sieve tube elements and companion cells originate from the same meristematic cell and are called sister cells.”
  3. Phloem Parenchyma:
     • Elongated living cells with cellulose walls and pits.
     • Store food materials, resins, latex, and mucilage.
     • “Phloem parenchyma is absent in most monocotyledons.”
  4. Phloem Fibres:
     • Sclerenchymatous, thick-walled, dead cells.
     • Also called bast fibres or extraxylary fibres.
     • “They are absent in primary phloem and present in secondary phloem.”
     • Commercial importance: Jute, flax, hemp, sunn hemp.
• Types of Primary Phloem:
  1. Protophloem: First formed phloem with narrow sieve tubes.
  2. Metaphloem: Later formed phloem with wider sieve tubes.

Differences in Gymnosperms and Angiosperms

• Gymnosperms:
  • Lack vessels in xylem.
  • Have albuminous cells and sieve cells (simple & different from sieve tube) in phloem.
  • “Albuminous cells perform functions similar to companion cells.”
  • Lack sieve tubes and companion cells in phloem.
• Angiosperms:
  • Xylem has vessels
  • Phloem has sieve tubes and companion cells

Examples of Tissue Functions

• Xylem: Water and mineral transport, mechanical strength.
  1. Tracheids and Vessels: Main water-transporting elements.
  2. Xylem Fibers: Provide mechanical support.
  3. Xylem Parenchyma: Food Storage and radial conduction
• Phloem: Food transport.
  1. Sieve Tubes and Companion Cells: Main food-transporting elements.
  2. Phloem Parenchyma: Stores food, resins, and latex.
  3. Phloem Fibers: Provide strength, used commercially (e.g., jute, flax).

Key Points

• Plant anatomy deals with internal structure and organization.
• Meristematic tissues are responsible for growth.
• Permanent tissues perform specific functions.
• Xylem and phloem are essential for transport and support.

The Tissue System

Types of Tissue Systems

• Plants have three types of tissue systems based on their location and structure:
  1. Epidermal Tissue System
  2. Ground or Fundamental Tissue System
  3. Vascular or Conducting Tissue System

“In flowering plants, different tissues work together as a functional unit called a tissue system.”

1. Epidermal Tissue System

• Location: Outermost layer of the plant body.
• Components:
  1. Epidermal Cells: Elongated, tightly packed, usually single-layered.
     • “Epidermal cells are parenchymatous with a thin lining of cytoplasm and a large central vacuole.”
     • Cuticle: Waxy layer on the outside (of epidermis), prevents water loss.
     • Cuticle is absent in roots.
  2. Stomata: Pores for transpiration and gas exchange, guarded by two guard cells.
     • Features:
     • Guard cells are bean-shaped in most plants
     • Guard cells are Dumb-bell shaped in grasses
     • Guard cells contain chloroplasts
     • “Inner walls of guard cells (towards the pore) are thick, while outer walls are thin.”
     • Function: “Guard cells regulate opening and closing of stomata.”
     • Subsidiary Cells: Specialized cells surrounding guard cells.
     • “Guard cells, subsidiary cells, and the stomatal pore together form the stomatal apparatus.“
     • “In dicot leaves, stomata are more numerous on the lower surface, while in monocot leaves they are almost equally distributed on both surfaces.”
  3. Epidermal Appendages: Trichomes and hairs.
     • Root Hairs: Unicellular, help absorb water and minerals.
     • Trichomes: Multicellular hairs on stems, can be branched or unbranched, soft or stiff, sometimes secretory.
     • “Trichomes help reduce water loss due to transpiration and provide protection.”

2. Ground Tissue System

• Components: All tissues except epidermis and vascular bundles.
• Types of Cells:
  • 1. Parenchyma 2. Collenchyma & 3. Sclerenchyma.
  • Locations: Found in cortex, hypodermis, endodermis, pericycle, medullary rays, and pith.
  • “It is the largest tissue system in the plant body.”
• Special Structure in Leaves:
  • Mesophyll: Thin-walled cells with chloroplasts for photosynthesis.

“Mesophyll is composed of chlorenchymatous cells and forms the ground tissue of leaves.”

3. Vascular Tissue System

• Components: Xylem and Phloem (together called vascular bundles).
• “The vascular tissue system forms a continuous network from roots to leaves.”
• Types of Vascular Bundles:
  1. Open Vascular Bundles: Found in dicot stems, Cambium present → secondary growth possible.
  2. Closed Vascular Bundles: Found in monocot stems, Cambium absent → no secondary growth.
• Arrangement of Vascular Bundles:
  1. Radial: Xylem and phloem alternate along different radii (common in roots).
  2. Conjoint: Xylem and phloem together along the same radius (common in stems and leaves), with phloem usually on the outer side of xylem.
  3. Concentric: One vascular tissue surrounds the other.

Subtypes:

“Conjoint vascular bundles may be collateral or bicollateral.”

“In collateral bundles, phloem lies on the outer side of xylem.”
“In bicollateral bundles, xylem is sandwiched between outer and inner phloem.”

“Concentric vascular bundles may be Amphicribal or Amphivasal.”

“Amphicribal (hadrocentric): phloem surrounds xylem (e.g., Prunus).”
“Amphivasal (leptocentric): xylem surrounds phloem (e.g., Dracaena, Yucca).”

Anatomy of Dicotyledonous and Monocotyledonous Plants

To understand plant tissue organization, we look at the transverse sections of roots, stems, and leaves.

a. Dicotyledonous Root

• Example: Sunflower root
• Structure:
  • Epiblema: Outermost layer with root hairs.
    • “Epiblema is a single-layered epidermis, also called uniseriate epidermis.”
    • “Root hairs are unicellular outgrowths of epidermal cells.”
    • “Hypodermis is absent in roots.”
  • Cortex: Layers of parenchyma cells with spaces.
    • “Cortex consists of several layers of thin-walled parenchyma cells with intercellular spaces.”
  • Endodermis: Single layer of barrel-shaped cells with Casparian strips (waxy).
    • “Casparian strips are present on radial and tangential walls of endodermal cells.”
  • Pericycle: Thick-walled cells, where lateral roots and vascular cambium start.
    • “Lateral roots arise from pericycle and are endogenous in origin.”
  • Vascular Bundles: 2-4 patches of xylem and phloem.
    • “Vascular bundles are radial in arrangement.”
    • “Xylem bundles are usually diarch to tetrarch and exarch in nature.”
    • “Xylem and phloem bundles are equal in number.”
  • Conjunctive Tissue: Basically a Parenchyma present between xylem and phloem.
  • Pith: Small or inconspicuous in Dicotyledonous Root.
    • “Pith is composed of parenchymatous cells and occupies the central region.”
  • Stele: “All tissues present internal to endodermis together form the stele.”
    • Includes pericycle, vascular bundles, and pith.

b. Monocotyledonous Root

• Similarities with Dicot Root: Has epidermis, cortex, endodermis, pericycle, vascular bundles, and pith.
• Differences:
  • More than six xylem bundles (polyarch).
  • Large, well-developed pith.
  • No secondary growth.

Additional Features:

“In older monocot roots, outer cortical layers become thick-walled and suberised, forming an exodermis.”
“Endodermal cells opposite protoxylem groups remain thin-walled and are called passage cells.”
“Passage cells help in inward and outward conduction of water and minerals.”

a. Dicotyledonous Stem

• Structure:
  • Epidermis: Outermost layer with cuticle, may have trichomes and stomata.
    • “Epidermis is single-layered (uniseriate) and covered with cuticle.”
    • “Many epidermal cells may form multicellular hairs called trichomes.”
  • Cortex: Three sub-zones:
    1. Hypodermis: “Hypodermis is multilayered and composed of collenchyma cells.”
       • “Cells may contain chloroplasts and provide mechanical support.”
    2. Middle Cortex: “Middle or general cortex is made of parenchyma with large intercellular spaces.”
    3. Endodermis: “Endodermis is single-layered and rich in starch grains, hence called starch sheath.”
  • Pericycle: Semi-lunar patches of sclerenchyma above phloem.
    • “Pericycle lies between endodermis and vascular bundles.”
    • “In sunflower, pericycle is heterogeneous with alternate patches of sclerenchyma and parenchyma.”
    • “In Cucurbita, pericycle is homogeneous and completely sclerenchymatous.”
  • Medullary Rays: “Medullary rays are composed of parenchyma cells.
    • Present between vascular bundles and help in lateral conduction.”
  • Vascular Bundles: Arranged in a ring, conjoint, collateral & open, with endarch protoxylem.
    • “Ring arrangement of vascular bundles is a characteristic feature of dicot stem.”
  • Pith: “Pith is well developed and consists of large parenchymatous cells with intercellular spaces.”

b. Monocotyledonous Stem

• Structure:
  • Epidermis: “Single-layered and covered with thick cuticle.”
    • “Epidermis generally lacks trichomes.”
  • Hypodermis: “Hypodermis is 2–3 layered and composed of sclerenchyma.”
    • “Provides mechanical support to the stem.”
  • Ground Tissue: “Ground tissue is not differentiated into cortex, endodermis, pericycle, pith and medullary rays.”
    • Thus Ground tissue is Large, parenchymatous.
  • Vascular Bundles: “Numerous vascular bundles are scattered throughout the ground tissue.”
    • “Each vascular bundle is conjoint, collateral and closed.”
    • “Each bundle is surrounded by a sclerenchymatous bundle sheath.”
  • Vascular Bundle Size: “Vascular bundles near the periphery are smaller than those towards the centre.”
  • Phloem Parenchyma: Absent.
  • Water Cavities: Present within vascular bundles.
    • “Protoxylem elements may break down to form a protoxylem lacuna (water cavity).”
    • “Protoxylem lacuna helps in storage of water.”

a. Dorsiventral (Dicotyledonous) Leaf

• Parts: Epidermis, Mesophyll, Vascular System
• Epidermis:
  • Epidermis is single-layered and covered by a thick cuticle.
  • Epidermis covers both upper (adaxial) and lower (abaxial) surfaces.
  • Abaxial epidermis has more stomata; adaxial may have none.
• Mesophyll:
  • “Mesophyll is composed of parenchyma cells containing chloroplasts.”
  • Mesophyll present between upper and lower epidermis.
  • Contains chloroplasts for photosynthesis.
  • Two types of cells:
    1. Palisade Parenchyma: Elongated, arranged vertically, located adaxially.
       • “Palisade cells are arranged parallel to each other and perpendicular to the leaf surface.”
    2. Spongy Parenchyma: Oval/round, loosely arranged, with air spaces/cavities, located below palisade cells.
• Vascular System:
  • Includes vascular bundles in veins and midrib.
  • Veins vary in thickness due to reticulate venation.
  • Vascular bundles are surrounded by thick-walled bundle sheath cells.
  • “Vascular bundles are conjoint, collateral and closed.”
  • “Xylem lies towards the upper epidermis and phloem towards the lower epidermis.”
  • “Bundle sheath of larger veins shows parenchymatous extensions towards both upper and lower epidermis.”
  • “Cambium is absent, hence secondary growth does not occur in leaves.”

b. Isobilateral (Monocotyledonous) Leaf

• Similarities to Dorsiventral Leaf: Has epidermis, mesophyll, and vascular bundles.
• Differences:
  • Stomata present on both surfaces.
    • “Such leaves are called amphistomatic leaves.”
  • Mesophyll is not divided into palisade and spongy parenchyma.
    • “Mesophyll remains uniform and undifferentiated.”
• Special Features in Grasses:
  • Bulliform Cells: Large, colorless cells along veins.
    • Function: Absorb water and become turgid to expose/flatten leaf surface.
    • Become flaccid during water stress to curl leaves inward and reduce water loss (transpiration).
• Vascular Bundles:
  • Venation:
    • “Parallel venation is reflected in almost equal-sized vascular bundles, except the main veins.”
    • “Distinct sclerenchymatous patches occur above and below large vascular bundles and extend up to the epidermis.”

Secondary Growth

“Increase in girth is called secondary growth and occurs after completion of primary growth.”

“Secondary growth is common in dicots and gymnosperms but absent in monocots.”

This growth involves two lateral meristems: vascular cambium and cork cambium.

1. Vascular Cambium

• Function: Produces new vascular tissues (xylem and phloem).
• Formation:
  • Initially present in patches between xylem and phloem.
  • Forms a continuous ring as intrafascicular cambium and interfascicular cambium cells join.

“Intrafascicular cambium is primary in origin, while interfascicular cambium is secondary in origin.”
“In dicot stem, vascular cambium is partly primary and partly secondary in origin.”

Cambial Ring Activity

• New Cells:
  • Inner side: Forms secondary xylem.
  • Outer side: Forms secondary phloem.
  • “Means, Cambium is more active on the inner side than on the outer side.”
• Secondary Xylem: More abundant, forms a compact mass.
• Secondary Phloem: Gets crushed over time.
  • “Means, Primary and secondary phloem are gradually crushed due to continuous formation of secondary xylem.”
• Medullary Rays: Narrow bands of parenchyma, extend radially.
  • “Secondary medullary rays conduct water and food in radial direction.”

Spring Wood and Autumn Wood

• Spring Wood:
  • Formed in spring.
  • Large vessels, lighter color, lower density.
• Autumn Wood:
  • Formed in winter.
  • Narrow vessels, darker color, higher density.
• Annual Rings: Alternating rings of spring and autumn wood indicate the tree’s age.
  • “Annual rings are formed due to unequal seasonal activity of vascular cambium.”

“Study of age determination of trees using annual rings is called dendrochronology.”

Heartwood and Sapwood

• Heartwood:
  • “Heartwood is also called duramen.”
  • Central, dark brown, and durable.
  • Contains tannins, resins, oils, etc.
  • Provides mechanical support, not involved in water conduction.
  • “Water conduction stops due to filling of vessels with organic substances and formation of tyloses.”
  • “Heartwood is resistant to fungi, insects, and termites.”
• Sapwood:
  • “Sapwood is also called alburnum.”
  • Peripheral, lighter in color.
  • “Sapwood contains living xylem elements.”
  • Conducts water and minerals from roots to leaves.

2. Cork Cambium

• “Cork cambium is also called phellogen and is a secondary lateral meristem.”
• Formation: Develops in the cortex region as the stem grows in girth.
• Structure: Made of narrow, rectangular cells.
• Function:
  • Outer Cells: Differentiate into cork (phellem), which is water-impervious due to suberin.
    • “Cork cells are dead and impermeable due to suberin deposition.”
  • Inner Cells: Differentiate into secondary cortex (phelloderm).
    • “Secondary cortex consists of living parenchymatous cells.”
• Periderm: Includes phellogen (cork cambium), phellem (cork), and phelloderm (secondary cortex).
  • “Periderm replaces epidermis during secondary growth.”
• Bark:
  • Comprises all tissues outside the vascular cambium, including secondary phloem and periderm.
  • Early/Soft Bark: Formed early in the season.
  • Late/Hard Bark: Formed later in the season.
• Lenticels:
  • “Lenticels are lens-shaped openings.”
  • Allow gas exchange between the atmosphere and internal tissues and reduce excessive transpiration.
  • Formed by phellogen producing parenchymatous cells instead of cork cells.
  • Common in woody trees.

Secondary Growth in Roots

• Origin: Vascular cambium in roots is secondary, originating from tissue below the phloem bundles and part of the pericycle.
• Formation:
  • Forms a continuous wavy ring that becomes circular.
  • Similar processes to secondary growth in stems.
• Occurrence: Seen in dicots and gymnosperms, but not in monocots.

“Root cambium is completely secondary in origin.”

Chapter Summary:

• A plant is made of different kinds of tissues.
• Plant tissues are classified into:
  • Meristematic: Apical, lateral, and intercalary.
  • Permanent: Simple and complex.
• “Meristematic tissues are responsible for growth due to continuous cell division, while permanent tissues perform specific functions after differentiation.”
• Functions of tissues:
  • Assimilate (process) food.
  • Store food.
  • Transport water and minerals.
  • Provide mechanical support.
• Three types of tissue systems:
  • Epidermal: Made of epidermal cells, stomata, and appendages.
    • “It forms the protective outer covering and regulates transpiration and gas exchange.”
  • Ground: Main bulk of the plant, divided into cortex, pericycle, and pith.
    • “It is mainly involved in storage, photosynthesis, and mechanical support.”
  • Vascular: Formed by xylem and phloem.
    • “It constitutes the conducting system of the plant.”
• Vascular bundles have different types based on cambium, xylem, and phloem.
• Vascular bundles transport water, minerals, and food.
  • “Arrangement of vascular bundles differs in roots, stems, and leaves.”
• Monocotyledonous and dicotyledonous plants have different internal structures.
  • They differ in type, number, and location of vascular bundles.
  • “These anatomical differences help in identification and classification of plants.”
• Secondary growth occurs in most dicot roots and stems.
  • Increases girth (diameter) of organs.
  • Involves vascular cambium and cork cambium.
  • “Secondary growth leads to formation of wood and bark.”
• Wood is secondary xylem.
  • Types of wood vary by composition and production time.
  • “Seasonal activity of cambium results in different types of wood such as spring wood and autumn wood.”