Anatomy of Flowering Plants

Given: We need to highlight anatomical differences through illustrations and descriptions.

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(a) Monocot Root vs. Dicot Root

| Feature | Monocot Root | Dicot Root |
|---|---|---|
| Number of xylem bundles (protoxylem) | Many (polyarch; 6 or more) | Few (2–6; diarch to hexarch) |
| Pith | Large and well-developed | Small or absent |
| Cortex | Broad | Broad |
| Endodermis | Prominent with Casparian strips | Prominent with Casparian strips |
| Pericycle | Single-layered | Single-layered (gives rise to lateral roots and vascular cambium) |
| Secondary growth | Absent | Present |
| Conjunctive tissue | Parenchymatous | Parenchymatous (later becomes cambium) |

Illustration description (T.S. Monocot Root):
From outside inward — Epidermis (with root hairs) → Cortex (multilayered parenchyma) → Endodermis (with Casparian strips) → Pericycle (single layer) → Many alternating xylem and phloem bundles arranged in a ring → Large central pith.

Illustration description (T.S. Dicot Root):
From outside inward — Epidermis → Cortex → Endodermis → Pericycle → 2–6 xylem bundles (exarch) alternating with phloem bundles → Small or no pith.

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(b) Monocot Stem vs. Dicot Stem

| Feature | Monocot Stem | Dicot Stem |
|---|---|---|
| Epidermis | Single layer, cuticle present | Single layer, cuticle present |
| Hypodermis | Sclerenchymatous | Collenchymatous |
| Ground tissue | Not differentiated into cortex and pith | Differentiated into cortex, endodermis, pericycle and pith |
| Vascular bundles | Scattered throughout ground tissue | Arranged in a ring |
| Bundle sheath | Sclerenchymatous (present) | Absent (or parenchymatous) |
| Bundle type | Conjoint, collateral, closed (no cambium) | Conjoint, collateral, open (cambium present) |
| Pith | Absent (not distinct) | Large, distinct |
| Secondary growth | Absent | Present |

Illustration description (T.S. Monocot Stem — e.g., maize):
Epidermis → Sclerenchymatous hypodermis → Undifferentiated ground tissue with vascular bundles scattered (more numerous and smaller towards periphery, larger towards centre) → Each vascular bundle oval-shaped, surrounded by sclerenchymatous bundle sheath, with phloem (including water-containing cavity) and xylem (Y-shaped metaxylem).

Illustration description (T.S. Dicot Stem — e.g., sunflower):
Epidermis (with trichomes) → Collenchymatous hypodermis → Cortex (parenchyma) → Endodermis (starch sheath) → Pericycle (sclerenchyma + parenchyma) → Ring of vascular bundles (each with xylem below, cambium in middle, phloem above) → Large central pith.

Conclusion: The key differences lie in the arrangement of vascular bundles (scattered vs. ringed), type of hypodermis, presence/absence of cambium, and differentiation of ground tissue.

Given: A transverse section (T.S.) of a young stem is observed under the microscope.

Concept: Monocot and dicot stems differ in several anatomical features visible in T.S.

Observations and Reasoning:

To ascertain whether the stem is monocot or dicot, observe the following features:

1. Hypodermis:
- If hypodermis is sclerenchymatous → Monocot stem
- If hypodermis is collenchymatous → Dicot stem

2. Arrangement of Vascular Bundles:
- If vascular bundles are scattered throughout the ground tissue → Monocot stem
- If vascular bundles are arranged in a ring (eustele) → Dicot stem

3. Ground Tissue:
- If ground tissue is not differentiated into cortex and pith → Monocot stem
- If ground tissue is differentiated into cortex, endodermis, pericycle and pith → Dicot stem

4. Bundle Sheath:
- If each vascular bundle is surrounded by a sclerenchymatous bundle sheath → Monocot stem
- If bundle sheath is absent or parenchymatous → Dicot stem

5. Cambium:
- If cambium is absent (closed vascular bundle) → Monocot stem
- If cambium is present between xylem and phloem (open vascular bundle) → Dicot stem

6. Pith:
- Absent or indistinct → Monocot stem
- Large and distinct → Dicot stem

Conclusion: By examining the above features — especially the arrangement of vascular bundles (scattered vs. in a ring) and the presence or absence of cambium — one can confidently identify the stem as monocot or dicot.

Given:
- Vascular bundles are conjoint, scattered, and surrounded by sclerenchymatous bundle sheath.
- Phloem parenchyma is absent.

Concept: These are characteristic anatomical features used to identify plant organs.

Identification:
The plant material is a Monocotyledonous stem (e.g., stem of maize/*Zea mays*).

Reasons:

1. Conjoint vascular bundles — xylem and phloem are present together in the same bundle, which is a feature of stems.

2. Scattered arrangement of vascular bundles throughout the ground tissue is the hallmark of monocot stems (atactostele).

3. Sclerenchymatous bundle sheath surrounding each vascular bundle is characteristic of monocot stems.

4. Absence of phloem parenchyma is a distinguishing feature of monocot stems; in dicot stems, phloem parenchyma is present.

Conclusion: The given plant material is a monocotyledonous stem.

Stomatal Apparatus:
The stomatal apparatus refers to the stomata along with the surrounding subsidiary (accessory) cells. It includes the guard cells, the stomatal pore between them, and the subsidiary cells (epidermal cells surrounding the guard cells).

Structure of Stomata:

Each stoma is composed of the following parts:

1. Stomatal Pore:
- A small opening (pore) present in the epidermis of leaves, young stems and other green parts.
- It allows gaseous exchange ($\text{CO}_2$, $\text{O}_2$, water vapour) between the plant and the atmosphere.

2. Guard Cells:
- Each stoma is surrounded by two guard cells.
- In dicots, guard cells are bean-shaped (kidney-shaped).
- In monocots (grasses), guard cells are dumbbell-shaped.
- Guard cells contain chloroplasts and can perform photosynthesis.
- The inner walls (facing the pore) of guard cells are thick and inelastic, while the outer walls are thin and elastic.
- This differential thickening causes the pore to open and close:
- When guard cells are turgid (absorb water) → outer walls bulge outward → inner walls curve → pore opens.
- When guard cells are flaccid (lose water) → walls straighten → pore closes.

3. Subsidiary (Accessory) Cells:
- Specialised epidermal cells surrounding the guard cells.
- They assist in the movement of guard cells.

4. Substomatal Chamber (Air cavity):
- A large intercellular space below the stomatal pore.
- Facilitates gaseous exchange with the mesophyll cells.

Labelled Diagram Description:

$$\boxed{\text{Stomatal Apparatus (T.S.)}}$$

The diagram should show:
- Epidermal cells on either side
- Two bean-shaped guard cells enclosing the stomatal pore
- Thick inner wall and thin outer wall of guard cells
- Chloroplasts inside guard cells
- Subsidiary cells flanking the guard cells
- Substomatal chamber below the pore

*(Note: Draw a neat labelled diagram showing the above components in transverse section and surface view.)*

Function: Stomata regulate transpiration and gaseous exchange. They open during the day (when guard cells are turgid due to photosynthesis-driven $\text{K}^+$ ion accumulation) and close at night.

Given: We need to name the three basic tissue systems and list the tissues under each.

Concept: According to Sachs (1875), the plant body is organised into three tissue systems based on their position and function.

The Three Basic Tissue Systems are:

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1. Epidermal Tissue System

This forms the outermost covering of the entire plant body.

Tissues included:
- Epidermis (epidermal cells)
- Stomata (guard cells and subsidiary cells)
- Epidermal appendages — trichomes (hair) and root hairs

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2. Ground Tissue System

This forms the main bulk of the plant body, lying between the epidermal and vascular tissue systems.

Tissues included:
- Parenchyma (in cortex, pith, mesophyll)
- Collenchyma (in hypodermis of dicot stems)
- Sclerenchyma (in hypodermis of monocot stems, bundle sheath)

It is divided into three zones:
- Cortex
- Pericycle
- Pith (medulla)

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3. Vascular Tissue System

This forms the conducting system of the plant.

Tissues included:
- Xylem (tracheids, vessels, xylem parenchyma, xylem fibres)
- Phloem (sieve tubes, companion cells, phloem parenchyma, phloem fibres)

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Summary Table:

| Tissue System | Tissues |
|---|---|
| Epidermal | Epidermis, stomata, trichomes, root hairs |
| Ground | Parenchyma, collenchyma, sclerenchyma |
| Vascular | Xylem, phloem |

Given: We need to explain the utility/importance of studying plant anatomy.

Answer:

The study of plant anatomy (internal structure of plants) is useful to us in the following ways:

1. Understanding Plant Functions:
Knowledge of internal structure helps us understand how plants carry out vital functions like photosynthesis, transpiration, conduction of water and food, and mechanical support.

2. Identification and Classification:
Anatomical features help in the identification and classification of plants. For example, the arrangement of vascular bundles, type of wood, and presence/absence of cambium help distinguish monocots from dicots.

3. Economic and Industrial Importance:
- Study of wood anatomy (secondary growth) helps in selecting timber for construction, furniture, and paper industries.
- Knowledge of fibres (jute, cotton, flax) helps in the textile industry.
- Understanding of latex-producing cells (laticifers) is useful in rubber production.

4. Agriculture and Horticulture:
- Understanding the anatomy of roots, stems and leaves helps in improving agricultural practices such as grafting, budding, and tissue culture.
- It helps in understanding the mechanism of absorption of water and minerals, which is important for irrigation and fertilisation.

5. Medicinal Uses:
- Anatomical studies help identify medicinal plants and their active parts (bark, roots, leaves).
- Adulteration of drugs can be detected through microscopic examination of plant tissues.

6. Palaeobotany:
Fossil plant anatomy helps in understanding the evolution of plants and past climatic conditions.

7. Forensic Science:
Plant anatomy is used in forensic investigations to identify plant materials found at crime scenes.

Conclusion: The study of plant anatomy is thus useful in diverse fields including botany, agriculture, industry, medicine, and forensic science.

Given: We need to describe the internal structure (anatomy) of a dorsiventral (dicotyledonous) leaf.

Definition: A dorsiventral leaf is one that has distinct upper (adaxial) and lower (abaxial) surfaces. It is characteristic of most dicotyledonous plants (e.g., mango, bean).

The T.S. of a dorsiventral leaf shows the following parts:

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1. Epidermis:

- The leaf is covered on both surfaces by a single layer of epidermal cells.
- Upper epidermis (adaxial): Covered by a thick cuticle; stomata are absent or very few.
- Lower epidermis (abaxial): Covered by a thin cuticle; stomata are more numerous.
- Epidermal cells are compactly arranged, lack chloroplasts (except guard cells).
- Guard cells are kidney-shaped and contain chloroplasts; they regulate opening and closing of stomata.

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2. Mesophyll:

The tissue between the upper and lower epidermis is called mesophyll. It is differentiated into two types:

(a) Palisade Parenchyma:
- Located just below the upper epidermis.
- Cells are elongated, cylindrical, arranged vertically and parallel to each other.
- Cells are compactly arranged with few intercellular spaces.
- Rich in chloroplasts → main site of photosynthesis.

(b) Spongy Parenchyma:
- Located below the palisade parenchyma and extends to the lower epidermis.
- Cells are oval or round, loosely arranged.
- Large intercellular spaces and air cavities are present → facilitate gaseous exchange.
- Fewer chloroplasts than palisade cells.

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3. Vascular System:

- Vascular bundles are present in the veins and midrib.
- Size of vascular bundles depends on the size of the vein (veins vary in thickness — reticulate venation).
- Each vascular bundle consists of:
- Xylem — located towards the upper (adaxial) surface.
- Phloem — located towards the lower (abaxial) surface.
- Vascular bundles are surrounded by a layer of thick-walled bundle sheath cells (parenchymatous).

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Labelled Diagram Description:

$$\boxed{\text{T.S. of Dorsiventral Leaf}}$$

The diagram should show (from top to bottom):
- Upper epidermis with cuticle (no stomata)
- Palisade parenchyma (elongated cells with chloroplasts)
- Spongy parenchyma (loosely arranged cells with air spaces)
- Lower epidermis with stomata and guard cells
- Midrib region showing vascular bundle with xylem (upper) and phloem (lower) surrounded by bundle sheath

*(Note: Draw a neat labelled diagram of T.S. of dorsiventral leaf showing all the above components.)*

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Summary Table:

| Component | Feature |
|---|---|
| Upper epidermis | Thick cuticle, no stomata |
| Palisade parenchyma | Elongated, compact, rich in chloroplasts |
| Spongy parenchyma | Loosely arranged, large air spaces |
| Lower epidermis | Thin cuticle, numerous stomata |
| Vascular bundle | Xylem (adaxial) + Phloem (abaxial), surrounded by bundle sheath |

Conclusion: The dorsiventral leaf is structurally adapted for efficient photosynthesis (palisade layer), gaseous exchange (spongy layer with air spaces), and conduction of water and food (vascular bundles).