Table of Contents (tap to open/close)
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.
Meristematic Tissues
- Meristems: Regions of active cell division in plants.
- Types of Meristems:
- Apical Meristems:
- Found at the tips of roots and shoots.
- Produce primary tissues.
- Root apical meristem is at the root tip.
- Shoot apical meristem is at the tip of the stem.
- Axillary buds form branches or flowers.
- Intercalary Meristems:
- Found between mature tissues.
- Occur in grasses.
- Help regenerate parts removed by grazing.
- Lateral Meristems:
- Found in mature regions of roots and shoots.
- Produce secondary tissues.
- Examples: vascular cambium, cork cambium.
- Apical Meristems:
- Primary Meristems: Appear early in the plant’s life.
- Secondary Meristems: Appear later and produce woody tissues.
- Cells from meristems become specialized and form permanent tissues.
Permanent Tissues
- Cells do not divide further.
- 2 Types: simple tissues and complex tissues.
Simple Tissues
- Made of one type of cell.
- Types of Simple Tissues:
- Parenchyma:
- Major component in organs.
- Cells are isodiametric (spherical, oval, round, polygonal, or elongated).
- Thin cell walls made of cellulose.
- Functions: photosynthesis, storage, secretion
- Collenchyma:
- Found below the epidermis in dicots.
- Cells thickened at corners due to cellulose, hemicellulose, and pectin.
- May contain chloroplasts for food assimilation.
- No intercellular spaces.
- Provides mechanical support to growing parts like young stems and leaf petioles.
- Sclerenchyma:
- Long, narrow cells with thick, lignified walls and pits.
- Usually dead and without protoplasts.
- Types: Fibres & Sclereids
- Fibres:
- Thick-walled, elongated, pointed cells.
- Found in groups in various plant parts.
- Sclereids:
- Spherical, oval, or cylindrical cells with very narrow cavities (lumen).
- Found in fruit walls of nuts, pulp of fruits like guava, pear, sapota, seed coats of legumes, and tea leaves.
- Fibres:
- Provides mechanical support to organs.
- Parenchyma:
Complex Tissues
- Complex Tissues: Made of more than one type of cells working together.
- Types: Xylem and Phloem.
Xylem
- Function: Conducts water and minerals from roots to stem and leaves, provides mechanical strength.
- Components:
- Tracheids: Elongated, tube-like, thick walls, dead, no protoplasm.
- Vessels: Long cylindrical tubes, lignified walls, no protoplasm, characteristic of angiosperms.
- Xylem Fibres: Thick walls, central lumen, can be septate or aseptate.
- Xylem Parenchyma: Living, thin-walled, stores food, aids radial water conduction.
- Types of Primary Xylem:
- Protoxylem: First formed, lies towards center (endarch in stems, exarch in roots).
- Metaxylem: Later formed, lies towards periphery in stems and center in roots.
Phloem
- Function: Transports food from leaves to other parts.
- Components:
- Sieve Tube Elements: Long, tube-like, perforated end walls (sieve plates), no nucleus.
- Companion Cells: Specialized parenchyma cells, control sieve tubes’ functions.
- Phloem Parenchyma: Elongated cells, store food, and other substances.
- Phloem Fibres: Sclerenchymatous cells, thick walls, used commercially (jute, flax, hemp).
- Types of Primary Phloem:
- Protophloem: Narrow sieve tubes, first formed.
- Metaphloem: Bigger sieve tubes, formed later.
Differences in Gymnosperms and Angiosperms
- Gymnosperms:
- Lack vessels in xylem.
- Have albuminous cells and sieve cells in phloem.
- Lack sieve tubes and companion cells in phloem.
- Angiosperms:
- Have vessels in xylem.
- Have sieve tubes and companion cells in phloem.
Examples of Tissue Functions
- Xylem: Water and mineral transport, mechanical strength.
- Tracheids and Vessels: Main water-transporting elements.
- Xylem Fibers: Provide strength.
- Xylem Parenchyma: Stores food, helps in water conduction.
- Phloem: Food transport.
- Sieve Tubes and Companion Cells: Main food-transporting elements.
- Phloem Parenchyma: Stores food, resins, and latex.
- Phloem Fibers: Provide strength, used commercially (e.g., jute, flax).
Key Points
- Plant anatomy shows the internal structure and organization.
- Tissues are crucial for plant growth and function.
- Meristematic tissues are for growth; permanent tissues are for specific functions.
- Xylem and phloem are vital for transport and support.
The Tissue System
Types of Tissue Systems
- Plants have three types of tissue systems based on their location and structure:
- Epidermal Tissue System
- Ground or Fundamental Tissue System
- Vascular or Conducting Tissue System
1. Epidermal Tissue System
- Location: Outermost layer of the plant body.
- Components:
- Epidermal Cells: Elongated, tightly packed, usually single-layered.
- Stomata: Pores for transpiration and gas exchange, guarded by two guard cells.
- Epidermal Appendages: Trichomes and hairs.
- Features:
- Cuticle: Waxy layer on the outside, prevents water loss (absent in roots).
- Guard Cells: Bean-shaped in most plants, dumb-bell shaped in grasses, have chloroplasts.
- Subsidiary Cells: Specialized cells around guard cells.
- Root Hairs: Unicellular, help absorb water and minerals.
- Trichomes: Multicellular hairs on stems, can be branched or unbranched, soft or stiff, sometimes secretory.
2. Ground Tissue System
- Components: All tissues except epidermis and vascular bundles.
- Types of Cells:
- Parenchyma: Found in cortex, pericycle, pith, medullary rays.
- Collenchyma and Sclerenchyma: Other types of cells present in this system.
- Special Structure in Leaves:
- Mesophyll: Thin-walled cells with chloroplasts for photosynthesis.
3. Vascular Tissue System
- Components: Xylem and Phloem (together called vascular bundles).
- Types of Vascular Bundles:
- Open Vascular Bundles: Found in dicot stems, have cambium, can form secondary tissues.
- Closed Vascular Bundles: Found in monocots, no cambium, can’t form secondary tissues.
- Arrangement:
- Radial: Xylem and phloem alternate along different radii (common in roots).
- Conjoint: Xylem and phloem together along the same radius (common in stems and leaves), with phloem usually on the outer side of xylem.
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.
- Cortex: Layers of parenchyma cells with spaces.
- Endodermis: Single layer of barrel-shaped cells with Casparian strips (waxy).
- Pericycle: Thick-walled cells, where lateral roots and vascular cambium start.
- Pith: Small or inconspicuous.
- Conjunctive Tissue: Parenchyma between xylem and phloem.
- Vascular Bundles: 2-4 patches of xylem and phloem.
- 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.
a. Dicotyledonous Stem
- Structure:
- Epidermis: Outermost layer with cuticle, may have trichomes and stomata.
- Cortex: Three sub-zones:
- Hypodermis: Collenchyma cells for strength.
- Middle Cortex: Parenchyma cells with spaces.
- Endodermis: Innermost layer, rich in starch (starch sheath).
- Pericycle: Semi-lunar patches of sclerenchyma above phloem.
- Medullary Rays: Parenchyma between vascular bundles.
- Vascular Bundles: Arranged in a ring, conjoint, open, with endarch protoxylem.
- Pith: Central part with parenchyma cells and spaces.
b. Monocotyledonous Stem
- Structure:
- Hypodermis: Sclerenchymatous for strength.
- Vascular Bundles: Scattered, each with a sclerenchymatous sheath, conjoint, and closed.
- Ground Tissue: Large, parenchymatous.
- Vascular Bundle Size: Peripheral ones smaller than central ones.
- Phloem Parenchyma: Absent.
- Water Cavities: Present within vascular bundles.
a. Dorsiventral (Dicotyledonous) Leaf
- Parts: Epidermis, Mesophyll, Vascular System
- Epidermis:
- Covers both upper (adaxial) and lower (abaxial) surfaces.
- Has a cuticle.
- Abaxial epidermis has more stomata; adaxial may have none.
- Mesophyll:
- Between upper and lower epidermis.
- Contains chloroplasts for photosynthesis.
- Two types of cells:
- Palisade Parenchyma: Elongated, arranged vertically, located adaxially.
- Spongy Parenchyma: Oval/round, loosely arranged, with air spaces, 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.
b. Isobilateral (Monocotyledonous) Leaf
- Similarities to Dorsiventral Leaf: Has epidermis, mesophyll, and vascular bundles.
- Differences:
- Stomata present on both surfaces.
- Mesophyll is not divided into palisade and spongy parenchyma.
- Special Features in Grasses:
- Bulliform Cells: Large, colorless cells along veins.
- Function: Absorb water and become turgid to expose leaf surface. Become flaccid during water stress to curl leaves and reduce water loss.
- Bulliform Cells: Large, colorless cells along veins.
- Venation:
- Parallel venation leads to nearly similar sizes of vascular bundles (except in main veins).
Secondary Growth
Secondary growth increases the girth of roots and stems in dicotyledonous plants. 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.
Cambial Ring Activity
- New Cells:
- Inner side: Forms secondary xylem.
- Outer side: Forms secondary phloem.
- Secondary Xylem: More abundant, forms a compact mass.
- Secondary Phloem: Gets crushed over time.
- Medullary Rays: Narrow bands of parenchyma, extend radially.
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.
Heartwood and Sapwood
- Heartwood:
- Central, dark brown, and durable.
- Contains tannins, resins, oils, etc.
- Provides mechanical support, not involved in water conduction.
- Sapwood:
- Peripheral, lighter in color.
- Conducts water and minerals from roots to leaves.
2. Cork Cambium
- 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.
- Inner Cells: Differentiate into secondary cortex (phelloderm).
- Periderm: Includes phellogen (cork cambium), phellem (cork), and phelloderm (secondary cortex).
- 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:
- Formed by phellogen producing parenchymatous cells instead of cork cells.
- Allow gas exchange between the atmosphere and internal tissues.
- 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.
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.
- 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.
- Ground: Main bulk of the plant, divided into cortex, pericycle, and pith.
- Vascular: Formed by xylem and phloem.
- Vascular bundles have different types based on cambium, xylem, and phloem.
- Vascular bundles transport water, minerals, and food.
- Monocotyledonous and dicotyledonous plants have different internal structures.
- They differ in type, number, and location of vascular bundles.
- Secondary growth occurs in most dicot roots and stems.
- Increases girth (diameter) of organs.
- Involves vascular cambium and cork cambium.
- Wood is secondary xylem.
- Types of wood vary by composition and production time.