Detailed notes- 2 – Chapter6-Anatomy Of Flowering Plants

6.2 ANATOMY OF DICOTYLEDONOUS AND MONOCOTYLEDONOUS PLANTS

1. Roots:

   • Transverse sections of roots reveal the arrangement of tissues such as epidermis, cortex, endodermis, pericycle, vascular tissues (xylem and phloem), and pith (in dicots).
   • This helps in understanding how roots absorb water and minerals from the soil, transport them upwards, and provide structural support to the plant.

2. Stems:

   • Transverse sections of stems illustrate the organization of tissues including epidermis, cortex, vascular bundles (xylem and phloem), pith (in dicots), and sometimes cambium (in dicots).
   • Examining these sections aids in comprehending how stems provide support to the plant, transport water and nutrients between roots and leaves, and undergo secondary growth (in dicots with cambium).

3. Leaves:

   • Transverse sections of leaves show the arrangement of tissues such as upper and lower epidermis, mesophyll (palisade and spongy layers), vascular bundles (veins), and sometimes stomata.
   • This helps in understanding how leaves carry out photosynthesis, exchange gases, and transport water and nutrients between roots and other parts of the plant.

4. Dicotyledonous vs. Monocotyledonous Plants:

   • Comparing transverse sections of these organs between dicotyledonous and monocotyledonous plants reveals differences in tissue organization.
   • For example, dicotyledonous plants typically have a distinct arrangement of vascular bundles in a ring pattern in stems, whereas monocotyledonous plants have scattered vascular bundles.
   • These comparisons provide insights into the structural and functional adaptations of different plant types to their respective environments.

6.2.1 Dicotyledonous Root 

1. Epiblema:

   • The outermost layer of the root.
   • Composed of epidermal cells.
   • Many epidermal cells protrude as unicellular root hairs, increasing surface area for absorption.

2. Cortex:

   • Consists of several layers of thin-walled parenchyma cells with intercellular spaces.
   • Parenchyma cells are involved in storage and transport of nutrients.

3. Endodermis:

   • The innermost layer of the cortex.
   • Comprises a single layer of barrel-shaped cells without intercellular spaces.
   • Tangential and radial walls of endodermal cells have deposits of water-impermeable, waxy material called suberin, forming Casparian strips.
   • Casparian strips regulate the movement of water and solutes into the vascular tissue.

4. Pericycle:

   • Lies next to the endodermis.
   • Consists of a few layers of thick-walled parenchymatous cells.
   • Initiates lateral roots and vascular cambium during secondary growth.

5. Pith:

   • Small or inconspicuous.
   • Parenchymatous cells may be present, but the pith is not well-developed.

6. Conjuctive Tissue:

   • Parenchymatous cells located between xylem and phloem.
   • Also known as conjunctive tissue or medullary rays.

7. Vascular Bundles:

   • Usually two to four patches of xylem and phloem.
   • Later, a cambium ring develops between xylem and phloem, enabling secondary growth.

8. Stele:

   • Consists of all tissues on the inner side of the endodermis, including pericycle, vascular bundles, and pith.
   • This central region of the root is collectively called the stele.

6.2.2 Monocotyledonous Root

1. Similarities:

   • Monocotyledonous roots share similarities with dicotyledonous roots in their basic structure.
   • Both have epidermis, cortex, endodermis, pericycle, vascular bundles, and pith.

2. Differences:

   • Number of Xylem Bundles:
     • In monocot roots, there are typically more than six (polyarch) xylem bundles.
     • Dicot roots generally have fewer xylem bundles.
   • Pith:
     • Monocot roots have a larger and well-developed pith compared to dicot roots.

3. Absence of Secondary Growth:

   • Unlike dicot roots, monocot roots do not undergo secondary growth.
   • Secondary growth involves the production of secondary tissues such as secondary xylem and phloem, which increase the girth of the root over time.
   • Due to the lack of cambium tissue, monocot roots maintain a constant diameter throughout their lifespan.

6.2.3 Dicotyledonous Stem

1. Epidermis:

   • Outermost protective layer of the stem.
   • Covered with a thin layer of cuticle.
   • May bear trichomes (epidermal hairs) and a few stomata for gas exchange.

2. Cortex:

   • The region between the epidermis and the pericycle.
   • Consists of three sub-zones:
     • Outer Hypodermis: Few layers of collenchymatous cells providing mechanical strength to the young stem.
     • Cortical Layers: Rounded thin-walled parenchymatous cells with conspicuous intercellular spaces.
     • Endodermis: Innermost layer of the cortex, rich in starch grains, also known as the starch sheath.

3. Pericycle:

   • Located on the inner side of the endodermis and above the phloem.
   • Present in the form of semi-lunar patches of sclerenchyma cells.

4. Medullary Rays:

   • Found between the vascular bundles.
   • Consist of a few layers of radially placed parenchymatous cells.

5. Vascular Bundles:

   • Arranged in a ring, a characteristic feature of dicot stems.
   • Each vascular bundle is conjoint (xylem and phloem together), open, and with endarch protoxylem (protoxylem located towards the center of the stem).

6. Pith:

   • Central portion of the stem, occupying a large area.
   • Consists of numerous rounded, parenchymatous cells with large intercellular spaces.

6.2.4 Monocotyledonous Stem

1. Sclerenchymatous Hypodermis:

   • Outermost layer beneath the epidermis.
   • Composed of sclerenchyma cells providing mechanical strength and support to the stem.

2. Vascular Bundles:

   • Large number of scattered vascular bundles throughout the stem.
   • Each vascular bundle is surrounded by a sclerenchymatous bundle sheath.
   • Vascular bundles are conjoint (xylem and phloem together) and closed (lack cambium for secondary growth).
   • Peripheral vascular bundles are generally smaller than centrally located ones.

3. Ground Tissue:

   • Large and conspicuous parenchymatous ground tissue.
   • Parenchyma cells make up the bulk of the stem’s interior, providing support and storing nutrients.

4. Phloem Parenchyma:

   • Absent in monocot stems.
   • Instead, water-containing cavities are present within the vascular bundles.

6.2.5 Dorsiventral (Dicotyledonous) Leaf

1. Epidermis:

   • Covers both upper (adaxial) and lower (abaxial) surfaces of the leaf.
   • Contains a conspicuous cuticle, which helps reduce water loss through transpiration.
   • Abaxial epidermis generally has more stomata than the adaxial epidermis. Adaxial epidermis may lack stomata altogether.

2. Mesophyll:

   • Tissue located between the upper and lower epidermis.
   • Possesses chloroplasts and carries out photosynthesis.
   • Comprised of parenchyma cells.
   • Two types of cells:
     • Palisade Parenchyma: Located adaxially, composed of elongated cells arranged vertically and parallel to each other for maximum light absorption.
     • Spongy Parenchyma: Situated below the palisade cells, composed of oval or round cells loosely arranged with large air spaces between them. Extends to the lower epidermis.

3. Vascular System:

   • Includes vascular bundles, which are visible in the veins and midrib of the leaf.
   • Size of vascular bundles depends on the size of the veins.
   • Veins vary in thickness in the reticulate venation pattern of dicot leaves.
   • Vascular bundles are surrounded by a layer of thick-walled bundle sheath cells.

6.2.6 Isobilateral (Monocotyledonous) Leaf

1. Epidermis:

   • Stomata are present on both the upper (adaxial) and lower (abaxial) surfaces of the epidermis.
   • This differs from dorsiventral leaves where stomata are usually more abundant on the lower surface.

2. Mesophyll:

   • Unlike dorsiventral leaves, the mesophyll in isobilateral leaves is not differentiated into palisade and spongy parenchyma.
   • Instead, the mesophyll is more uniform in structure throughout the leaf.

3. Specialized Cells in Grasses:

   • In grasses, certain adaxial epidermal cells along the veins undergo modification into large, empty, colorless cells known as bulliform cells.
   • Bulliform cells play a role in leaf movement: when turgid, they expose the leaf surface, and when flaccid due to water stress, they cause the leaves to curl inwards, minimizing water loss.

4. Vascular Bundles:

   • In monocot leaves, the parallel venation pattern is reflected in the nearly similar sizes of vascular bundles, except in main veins.
   • This differs from the reticulate venation pattern seen in dicot leaves, where the size of vascular bundles may vary more noticeably.