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Chapter 6 Anatomy Of Flowering Plants
While the external morphology of higher plants (angiosperms) shows great diversity, their internal structure (**anatomy**) also reveals similarities and differences, especially between monocots and dicots. The study of the internal structure of plants is called anatomy. Plants are organized from basic units (cells) into tissues, and tissues into organs. Different organs (roots, stems, leaves) have distinct internal structures, reflecting their specialized functions and adaptations to various environments.
The Tissue System
Tissues are groups of cells with similar structure and function. Based on their location in the plant body and their structure and function, tissues are organized into three types of tissue systems.
The Epidermal Tissue System
The **epidermal tissue system** forms the outermost covering of the entire plant body. It acts as a protective layer.
Components of the epidermal tissue system:
- **Epidermal cells:** Elongated, compactly arranged cells forming a continuous outermost layer (epidermis) of the primary plant body. Usually single-layered. They are parenchymatous with a small amount of cytoplasm, a large vacuole, and a prominent nucleus.
- **Cuticle:** A waxy, thick layer covering the outer surface of the epidermis in aerial parts (stems, leaves). It helps prevent water loss through transpiration. Cuticle is absent in roots.
- **Stomata:** Structures present in the epidermis of leaves (and sometimes stems). They regulate transpiration (water vapour loss) and gaseous exchange (CO$_2$ intake, O$_2$ release). Each stoma is composed of:
- **Stomatal pore:** An opening.
- **Guard cells:** Two bean-shaped (in dicots) or dumb-bell shaped (in grasses/monocots) cells surrounding the stomatal pore. They contain chloroplasts and regulate stomatal opening and closing by changing turgor. Outer walls are thin, inner walls (towards pore) are thickened.
- **Subsidiary cells:** Sometimes a few epidermal cells near the guard cells become specialized in shape and size, forming part of the stomatal apparatus.
The stomatal aperture, guard cells, and surrounding subsidiary cells are collectively called the **stomatal apparatus**.
- **Epidermal appendages (Hairs):** Extensions of epidermal cells.
- **Root hairs:** Unicellular elongations of epidermal cells in the root, specialized for absorbing water and minerals from the soil.
- **Trichomes:** Epidermal hairs on the stem and shoot system. Usually multicellular, branched or unbranched, soft or stiff, sometimes secretory. Help prevent water loss by transpiration.
Question 4. What is stomatal apparatus? Explain the structure of stomata with a labelled diagram.
Answer:
The **stomatal apparatus** is the structure found in the epidermis of leaves (and young stems) that facilitates gaseous exchange (like CO$_2$ and O$_2$) and transpiration (loss of water vapor). It consists of the stomatal aperture, the guard cells, and the surrounding subsidiary cells (if present).
Structure of stomata:
Each stoma is composed of:
- **Stomatal pore:** An opening through which gases diffuse.
- **Guard cells:** Two specialized epidermal cells that surround the stomatal pore. In dicots, they are typically bean-shaped. In monocots (like grasses), they are dumb-bell shaped. Guard cells contain chloroplasts and can change their shape by altering their turgor pressure, which opens or closes the stomatal pore. The inner walls (facing the pore) are thick, while the outer walls are thin.
- **Subsidiary cells (sometimes):** Some epidermal cells adjacent to the guard cells may be differentiated in size and shape and are termed subsidiary cells.
Labelled diagram of stomata:
The Ground Tissue System
The **ground tissue system** includes all plant tissues except the epidermal tissue system and the vascular bundles. It forms the main bulk of the plant body.
It is composed of simple tissues such as:
- **Parenchyma:** Thin-walled cells, usually present in the cortex, pericycle, pith, and medullary rays of primary stems and roots. In leaves, the parenchymatous cells containing chloroplasts form the **mesophyll** tissue, which is involved in photosynthesis. Parenchyma is involved in various functions like storage, photosynthesis, secretion, etc.
- **Collenchyma:** Living cells, often found below the epidermis in dicot stems, providing mechanical support to young growing parts.
- **Sclerenchyma:** Dead cells with lignified walls, providing mechanical strength and support.
The ground tissue system can often be differentiated into regions like the cortex (between epidermis and vascular bundles), pericycle (inner to endodermis), pith (central region), and medullary rays (parenchyma between vascular bundles in stems).
The Vascular Tissue System
The **vascular tissue system** consists of the complex tissues **xylem** and **phloem**. These tissues are responsible for the transport of water, minerals, and food (photosynthates) throughout the plant body. Xylem and phloem are typically grouped together in structures called **vascular bundles**.
Types of vascular bundles based on cambium presence and arrangement of xylem and phloem:
- **Open vascular bundles:** **Cambium** (a type of meristematic tissue) is present between the xylem and phloem. The presence of cambium allows these bundles to form secondary xylem and phloem (secondary growth, leading to increase in girth). Characteristic of **dicot stems and roots**.
- **Closed vascular bundles:** **Cambium is absent** between xylem and phloem. These bundles lack the ability to form secondary tissues, so monocots (which primarily have closed bundles) usually do not undergo secondary growth. Characteristic of **monocot stems and leaves**.
- **Radial arrangement of vascular bundles:** Xylem and phloem are arranged in an **alternate manner along different radii**. Characteristic of **roots**.
- **Conjoint arrangement of vascular bundles:** Xylem and phloem are jointly situated along the **same radius**. Common in **stems and leaves**. In conjoint bundles, phloem is usually located on the outer side of xylem.
Question 5. Name the three basic tissue systems in the flowering plants. Give the tissue names under each system.
Answer:
The three basic tissue systems in flowering plants are:
- **Epidermal Tissue System:** Includes epidermal cells, stomata, and epidermal appendages (root hairs and trichomes).
- **Ground Tissue System:** Includes all tissues except the epidermis and vascular bundles. It is composed of simple tissues: parenchyma, collenchyma, and sclerenchyma. (It can be further differentiated into cortex, pericycle, pith, medullary rays, mesophyll).
- **Vascular Tissue System:** Includes complex tissues: xylem and phloem. These form vascular bundles.
Anatomy Of Dicotyledonous And Monocotyledonous Plants
Dicotyledonous (dicot) and Monocotyledonous (monocot) plants, while sharing basic anatomical components, show distinct differences in the arrangement and characteristics of their tissues, particularly in roots, stems, and leaves.
Studying transverse sections (T.S.) of mature zones of these organs helps understand their internal organization.
Dicotyledonous Root
T.S. of a dicot root (e.g., sunflower root):
- **Epiblema:** Outermost layer, protective. Bears unicellular root hairs for water/mineral absorption.
- **Cortex:** Consists of several layers of thin-walled parenchyma cells with intercellular spaces, located below epiblema. Involved in storage.
- **Endodermis:** Innermost layer of the cortex. Single layer of barrel-shaped cells with no intercellular spaces. Characteristic **Casparian strips** (suberin deposition) on tangential and radial walls, which regulate water/mineral movement into the vascular cylinder.
- **Pericycle:** Few layers of thick-walled parenchymatous cells located next to endodermis (towards the center). Site of initiation of lateral roots and vascular cambium during secondary growth.
- **Vascular bundles:** Xylem and phloem patches are arranged in a **radial manner** (alternate along different radii). Typically **two to four** (diarch to tetrarch) xylem and phloem bundles. Conjuctive tissue (parenchyma) lies between xylem and phloem.
- **Pith:** Central region. Small or inconspicuous.
- **Stele:** All tissues inside the endodermis (pericycle, vascular bundles, pith) constitute the stele.
Dicot roots undergo secondary growth (increase in girth).
Monocotyledonous Root
Anatomy is similar to dicot root but with some key differences:
- Has epidermis (epiblema) with unicellular root hairs, cortex, endodermis with Casparian strips, and pericycle.
- **Vascular bundles:** Xylem and phloem are arranged radially. Have usually **more than six** (polyarch) xylem bundles (whereas dicots typically have 2-4).
- **Pith:** Central region. **Large and well developed**.
Monocot roots **do not undergo secondary growth**.
Dicotyledonous Stem
T.S. of a typical young dicot stem:
- **Epidermis:** Outermost protective layer. Covered with thin cuticle. May bear trichomes and few stomata.
- **Cortex:** Multiple layers of cells between epidermis and pericycle. Differentiated into:
- **Hypodermis:** Outer few layers just below epidermis. Consist of **collenchymatous** cells, providing mechanical strength to young stem.
- Cortical layers: Below hypodermis, consist of rounded, thin-walled parenchyma cells with intercellular spaces.
- **Endodermis:** Innermost layer of cortex. Cells are rich in starch grains (starch sheath).
- **Pericycle:** On the inner side of endodermis, above phloem. Forms semi-lunar patches of **sclerenchyma**.
- **Vascular bundles:** Arranged in a **ring**. Conjoint and **open** (cambium present). Usually 2-4 xylem and phloem patches (dicots typically have definite number of vascular bundles in a ring). **Endarch protoxylem** (protoxylem towards the center, metaxylem towards periphery). Medullary rays (parenchyma) between vascular bundles.
- **Pith:** Large number of rounded, parenchymatous cells with large intercellular spaces, occupying the central portion. Forms the central pith region.
Dicot stems undergo secondary growth (increase in girth) due to vascular cambium and cork cambium.
Monocotyledonous Stem
T.S. of a typical monocot stem (e.g., maize):
- **Epidermis:** Outermost protective layer. Cuticle present.
- **Hypodermis:** Sclerenchymatous. Provides mechanical strength.
- **Ground tissue:** No differentiation into cortex, endodermis, pericycle, or pith. A large, conspicuous parenchymatous ground tissue extends throughout the stem, enclosing scattered vascular bundles.
- **Vascular bundles:** **Scattered** throughout the ground tissue (not arranged in a ring). Conjoint and **closed** (cambium absent). Each bundle is surrounded by a **sclerenchymatous bundle sheath**. Peripheral bundles are generally smaller than centrally located ones. Phloem parenchyma is absent. Water-containing cavities are present within vascular bundles.
Monocot stems **do not undergo secondary growth**.
Question 2. Cut a transverse section of young stem of a plant from your school garden and observe it under the microscope. How would you ascertain whether it is a monocot stem or a dicot stem? Give reasons.
Answer:
To ascertain if a young stem is monocot or dicot, I would look for the following anatomical features in its transverse section under a microscope and compare them:
| Feature | Dicot Stem | Monocot Stem |
|---|---|---|
| Hypodermis | Often collenchymatous. | Often sclerenchymatous. |
| Ground Tissue | Differentiated into cortex, endodermis, pericycle, medullary rays, and pith. | Undifferentiated into distinct regions; a uniform parenchymatous ground tissue. |
| Vascular Bundles Arrangement | Arranged in a ring. | Scattered throughout the ground tissue. |
| Vascular Bundles Type | Conjoint and Open (cambium present). | Conjoint and Closed (cambium absent). |
| Bundle Sheath | Bundle sheath is less prominent or absent (vascular bundles often surrounded by sclerenchymatous pericycle patches). | Each vascular bundle is surrounded by a prominent sclerenchymatous bundle sheath. |
| Pith | Large, centrally located pith region. | Pith region is not distinct due to scattered bundles; central bundles might be larger. |
| Secondary Growth | Undergoes secondary growth (increase in girth) due to cambium. | Does not undergo secondary growth (cambium absent). |
Reasons: Observing these distinct features, particularly the **arrangement and type of vascular bundles**, the differentiation of the ground tissue, and the presence or absence of cambium (though harder to spot directly in a young stem T.S. without experience, the 'open' bundle type implies its presence), allows me to confidently classify the stem as either monocot or dicot.
Question 3. The transverse section of a plant material shows the following anatomical features - (a) the vascular bundles are conjoint, scattered and surrounded by a sclerenchymatous bundle sheaths. (b) phloem parenchyma is absent. What will you identify it as?
Answer:
Based on the given anatomical features:
- Vascular bundles are conjoint (xylem and phloem on the same radius) and scattered (not in a ring). Scattered vascular bundles are characteristic of monocots. Conjoint bundles are characteristic of stems and leaves.
- Vascular bundles are surrounded by sclerenchymatous bundle sheaths. This is characteristic of monocot stems.
- Phloem parenchyma is absent. This is also a characteristic feature often seen in monocot stems.
Considering these features together (scattered conjoint vascular bundles with bundle sheaths and absence of phloem parenchyma), I will identify the plant material as a **monocotyledonous stem**.
Question 6. How is the study of plant anatomy useful to us?
Answer:
The study of plant anatomy is useful to us in many ways:
- **Classification:** Anatomical features provide crucial information for the classification and identification of plants, especially distinguishing between monocots and dicots and different taxa.
- **Understanding Function:** Anatomy reveals how the internal structure of plant parts is related to their specific functions (e.g., xylem/phloem for transport, mesophyll for photosynthesis, protective tissues).
- **Adaptations:** It helps understand how plants are adapted to diverse environments (e.g., adaptations in leaves and stems of desert plants to reduce water loss).
- **Agriculture and Forestry:** Knowledge of plant anatomy is essential for understanding plant growth, development, propagation techniques, and disease resistance, which is vital for improving yields in agriculture and forestry.
- **Resource Utilization:** Understanding wood structure (secondary xylem anatomy) is important for its various uses in construction, furniture, and paper industries.
- **Pharmacognosy:** Anatomy helps in the identification and quality control of crude drugs obtained from plants.
Dorsiventral (Dicotyledonous) Leaf
A **dorsiventral leaf** (typical of dicots) has distinct upper (adaxial) and lower (abaxial) surfaces, differing in structure and appearance.
T.S. through the lamina shows three main parts:
- **Epidermis:** Covers both upper (adaxial) and lower (abaxial) surfaces, with a conspicuous cuticle. Abaxial epidermis usually has more stomata than the adaxial epidermis (sometimes none on adaxial).
- **Mesophyll:** Tissue between upper and lower epidermis. Contains chloroplasts and carries out photosynthesis. Made of parenchyma cells differentiated into two types:
- **Palisade parenchyma:** Adaxially placed, made of elongated cells arranged vertically and parallel, densely packed with chloroplasts.
- **Spongy parenchyma:** Situated below palisade cells, consists of oval or round, loosely arranged cells with large intercellular spaces and air cavities.
- **Vascular system:** Includes vascular bundles in veins and the midrib. Size depends on vein size. Surrounded by thick-walled **bundle sheath cells**. In dicot leaves, vascular bundles are conjoint and open (though cambium is usually non-functional). Xylem is usually located towards the adaxial side, phloem towards the abaxial side. Reticulate venation is characteristic.
Isobilateral (Monocotyledonous) Leaf
An **isobilateral leaf** (typical of monocots, e.g., grasses) has upper and lower surfaces that are similar in appearance.
Anatomy is similar to a dorsiventral leaf but with characteristic differences:
- **Stomata:** Present on **both** adaxial and abaxial surfaces.
- **Mesophyll:** Not differentiated into palisade and spongy parenchyma; consists of uniformly shaped parenchyma cells with intercellular spaces.
- **Vascular bundles:** Conjoint and closed. Vary in size depending on the vein. Parallel venation is characteristic, reflected in the near-similar size of vascular bundles (except main veins). Surrounded by bundle sheath cells.
- **Bulliform cells:** In grasses, certain adaxial epidermal cells along the veins are modified into large, empty, colourless cells. They help in minimizing water loss by causing leaves to curl inwards when flaccid due to water stress.
Question 7. Describe the internal structure of a dorsiventral leaf with the help of labelled diagrams.
Answer:
The internal structure of a dorsiventral (dicotyledonous) leaf, as seen in a transverse section, is differentiated into three main parts:
- **Epidermis:** Forms the outermost protective layers covering both the upper (adaxial) and lower (abaxial) surfaces of the leaf. It is usually a single layer of cells. A prominent waxy cuticle covers both epidermal layers to prevent water loss. The lower (abaxial) epidermis generally bears more stomata than the upper (adaxial) epidermis. Stomata are pores surrounded by guard cells that regulate gas exchange.
- **Mesophyll:** The tissue located between the upper and lower epidermis. It is the primary site of photosynthesis due to the presence of chloroplasts in its cells. In dorsiventral leaves, the mesophyll is differentiated into two distinct types of parenchyma cells:
- **Palisade parenchyma:** Located below the upper epidermis. Consists of elongated, vertically arranged, closely packed cells with abundant chloroplasts.
- **Spongy parenchyma:** Located below the palisade parenchyma and extends to the lower epidermis. Consists of irregularly shaped or rounded, loosely arranged cells with large intercellular spaces and air cavities. These spaces facilitate gas exchange within the leaf.
- **Vascular System:** Consists of vascular bundles found in the veins and the midrib of the leaf. Vascular bundles contain xylem (usually towards the upper/adaxial side) and phloem (usually towards the lower/abaxial side), responsible for transport. The size of the vascular bundles varies with the size of the veins. Each vascular bundle is typically surrounded by a layer of thick-walled cells called a **bundle sheath**.
Labelled diagram of T.S. of a dorsiventral leaf:
Exercises
Question 1. Draw illustrations to bring out the anatomical difference between
(a) Monocot root and Dicot root
(b) Monocot stem and Dicot stem
Answer:
Question 2. Cut a transverse section of young stem of a plant from your school garden and observe it under the microscope. How would you ascertain whether it is a monocot stem or a dicot stem? Give reasons.
Answer:
Question 3. The transverse section of a plant material shows the following anatomical features - (a) the vascular bundles are conjoint, scattered and surrounded by a sclerenchymatous bundle sheaths. (b) phloem parenchyma is absent. What will you identify it as?
Answer:
Question 4. What is stomatal apparatus? Explain the structure of stomata with a labelled diagram.
Answer:
Question 5. Name the three basic tissue systems in the flowering plants. Give the tissue names under each system.
Answer:
Question 6. How is the study of plant anatomy useful to us?
Answer:
Question 7. Describe the internal structure of a dorsiventral leaf with the help of labelled diagrams.
Answer: