Chapter no 8: Plant tissue and Anatomy

 

Plant Tissues and Anatomy

Anatomy is the study of the internal structure of an organism. Plants are made of organs like root, stem, and leaf. Each organ is made of groups of cells. A group of similar or different cells having a common function and origin is called a tissue.

Plant tissues are divided into two main types:

• Meristematic tissue
• Permanent tissue

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8.2 Meristematic Tissue

Meristematic tissue is a group of young, living cells that have the ability to divide. These tissues are found in growing regions of the plant.

• Cells are small and actively dividing
• Cells are polygonal or round in shape
• No intercellular spaces are present
• Cell wall is thin and made of cellulose
• Nucleus is large and prominent
• Vacuoles are very small or absent
• Cells have high metabolic activity
• Cells are immature

Classification of Meristem:

1. Based on Origin

• Promeristem (Primordial meristem) – found at root and shoot tips in embryo.
• Primary meristem – found in root and shoot tips, forms primary tissues.
• Secondary meristem – develops from permanent tissues later (e.g. cambium).

2. Based on Position

• Apical meristem – present at tips of root and shoot, increases length.
• Intercalary meristem – present at nodes, common in grasses.
• Lateral meristem – present on sides, increases thickness (e.g. cambium).

3. Based on Function

• Protoderm – forms epidermis.
• Procambium – forms vascular tissues.
• Ground meristem – forms cortex, pith, endodermis etc.

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8.3 Permanent Tissue

Permanent tissues are tissues that have lost the ability to divide and have fixed shape and size.

A. Simple Permanent Tissues

1. Parenchyma

• Thin-walled living cells
• Large vacuole and nucleus present
• Intercellular spaces present
• Found in cortex, pith, leaves, fruits, seeds
• Functions: food storage, photosynthesis, gas exchange, buoyancy

2. Collenchyma

• Living tissue with unevenly thickened cell walls
• Cells are compact without intercellular spaces
• Provides mechanical support
• Allows bending of stem and leaves
• Found in young stems and leaf petiole

3. Sclerenchyma

• Dead tissue with thick lignified walls
• Two types: fibres and sclereids
• Provides strength and rigidity
• Commercial fibres like jute, flax, hemp come from this tissue

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B. Complex Permanent Tissues

1. Xylem

Xylem is a dead tissue that transports water and minerals from roots to other parts.

• Tracheids
• Vessels
• Xylem parenchyma
• Xylem fibres

Xylem also provides mechanical support.

2. Phloem

Phloem is a living tissue that transports food from leaves to other parts of the plant.

• Sieve tubes
• Companion cells
• Phloem parenchyma
• Phloem fibres

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8.4 Tissue Systems

A. Epidermal Tissue System

• Outermost protective layer
• Covered with cuticle to prevent water loss
• Has root hairs for absorption
• Has trichomes on stem
• Has stomata for gas exchange

B. Ground Tissue System

Includes cortex, pith, pericycle, mesophyll and hypodermis. It is mainly made of parenchyma, collenchyma and sclerenchyma.

C. Vascular Tissue System

Includes xylem and phloem arranged as vascular bundles.

• Radial bundles in roots
• Conjoint collateral bundles in stems
• Open bundles (with cambium) in dicots
• Closed bundles (without cambium) in monocots

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8.5 Secondary Growth in Plants

Primary growth increases length. Secondary growth increases thickness of stem and root.

In dicots and gymnosperms, cambium forms a ring called cambial ring. Cambium produces:

• Secondary xylem (inside)
• Secondary phloem (outside)

Secondary tissues provide support, protection and conduction.

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8.6 Wood

• Spring wood: Light color, wide vessels, low density
• Autumn wood: Dark color, thick walls, high density
• Heartwood: Inner non-functional hard wood
• Sapwood: Outer functional conducting wood

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8.7 Cork Cambium and Secondary Growth

Cork cambium (phellogen) forms cork (phellem) outside and phelloderm inside. Together they form periderm.

• Cork prevents water loss
• Lenticels help in gas exchange
• Bark is all tissues outside cambium

Monocots generally do not show secondary growth.

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8.8 Anatomy of Root, Stem and Leaf

A. Anatomy of Dicot Root

• Epiblema with root hairs
• Cortex for storage
• Endodermis with Casparian strips
• Pericycle forms lateral roots
• Radial vascular bundles
• Pith present

B. Anatomy of Monocot Root

• Polyarch xylem
• Large pith
• No secondary growth

C. Anatomy of Dicot Stem

• Epidermis with trichomes
• Hypodermis of collenchyma
• Cortex and endodermis
• Ring of vascular bundles
• Large pith

D. Anatomy of Monocot Stem

• No trichomes
• Scattered vascular bundles
• Bundle sheath present
• No cambium and no secondary growth

E. Anatomy of Dicot Leaf (Dorsiventral)

• Upper epidermis with cuticle
• Palisade mesophyll for photosynthesis
• Spongy mesophyll for gas exchange
• Vascular bundles with xylem and phloem
• Lower epidermis with stomata

F. Isobilateral Leaf (Monocot Leaf)

• Both surfaces similar
• Stomata on both sides
• No palisade and spongy differentiation
• Parallel venation