Anatomy of Plant Parts

ANATOMY OF PLANT PARTS

  •      Understanding the internal structure of plant organs, such as roots, stems, and leaves, is essential for comprehending tissue organization. 
  •     To grasp this concept, let's examine the transverse section of a mature zone of these organs. 

Dicotyledonous root

1. Epiblema (Root Epidermis):

- Outermost layer of the root.

- Epiblema cells often extend as unicellular root hairs, aiding in water and nutrient absorption.

 

2. Cortex:

- Comprises multiple layers of thin-walled parenchyma cells with intercellular spaces.

- Stores food, facilitates gas exchange and supports the root structure.

 

3. Endodermis:

- Innermost layer of the cortex.

- Consists of a single layer of barrel-shaped cells with no intercellular spaces.

- Casparian strips made of water-impermeable, waxy suberin are present on the tangential and radial walls of endodermal cells.

- Acts as a barrier that controls the entry of water and minerals into the vascular cylinder.

 

4. Pericycle:

- A few layers of thick-walled parenchymatous cells located adjacent to the endodermis.

- Initiates the formation of lateral roots and the vascular cambium during secondary growth.

 

5. Pith:

- Small or inconspicuous in dicot roots.

- Limited role, primarily providing structural support.

 

6. Conjunctive Tissue:

- Parenchymatous cells lying between the xylem and phloem.

- Supports and connects the vascular tissues.

- Vascular Bundles: Typically, two to four patches of xylem and phloem exist within this tissue.

- Protoxylem is the first-formed, smaller, and thinner-walled xylem tissue in plants, while metaxylem is the second-formed, larger, and thicker-walled xylem tissue. Both transport water and nutrients, but the metaxylem provides more support in mature plants.

 

7. Stele:

- Includes all tissues on the inner side of the endodermis, such as pericycle, vascular bundles, and pith.

- Houses the central cylinder of vascular tissues responsible for the transport of water, nutrients, and food throughout the root.

 

 

The anatomy of dicotyledonous roots is characterized by distinct layers and tissues that collectively enable water and nutrient uptake, structural support, and secondary growth.

 

Monocotyledonous root

- The anatomy of monocotyledonous (monocot) roots shares several similarities with dicotyledonous (dicot) roots, but there are key differences to note.

Similarities to Dicot Roots:

1. Epidermis:

The outermost layer of the root provides protection.

 

2. Cortex:

Comprising parenchyma cells with intercellular spaces, serving functions like storage and support.

 

3. Endodermis:

The innermost cortex layer with barrel-shaped cells and Casparian strips, regulates water and mineral entry.

 

4. Pericycle:

A layer of parenchymatous cells adjacent to the endodermis, is involved in lateral root formation.

 

5. Vascular Bundles:

Present in the root's central region, responsible for transporting water and nutrients.

Differences from Dicot Roots:

1. Number of Xylem Bundles:

In monocot roots, there are typically more than six xylem bundles (polyarch), whereas dicot roots usually have fewer.

 

2. Pith:

Monocot roots have a large and well-developed pith, which is often absent or inconspicuous in dicot roots.

 

3. Secondary Growth:

Unlike dicot roots, monocotyledonous roots do not undergo secondary growth, which means they do not produce secondary tissues like secondary xylem or phloem.

 

 

Dicotyledonous stem

- A transverse section of a typical young dicotyledonous stem reveals several distinct layers and tissues that perform various functions to support the plant's growth and structure.

 

1. Epidermis:

- Outermost protective layer covered with a thin cuticle.

- May bear trichomes (hairs) and a few stomata for gas exchange.

- Protects the stem from physical damage and minimizes water loss.

 

2. Cortex:

- Tissues are arranged in multiple layers between the epidermis and the pericycle.

-Subzones:

a.    Hypodermis:

A few layers of collenchymatous cells just below the epidermis, provide mechanical strength to the young stem.

b.    Parenchymatous Layers:

Comprising rounded, thin-walled parenchyma cells with visible intercellular spaces.

c.    Endodermis (Starch Sheath):

The innermost layer of the cortex is rich in starch grains.

- Function: Support, storage, and protection.

 

3. Pericycle:

- Present on the inner side of the endodermis and above the phloem.

- Semilunar patches of sclerenchyma.

- Provides mechanical support and protection to vascular tissues.

 

4. Medullary Rays:

- A few layers of radially arranged parenchymatous cells located between vascular bundles.

- Facilitates lateral transport of water, nutrients, and support.

 

5. Vascular Bundles:

- Numerous vascular bundles form a characteristic "ring" arrangement in dicot stems.

- Each vascular bundle is conjoint, open, and contains endarch protoxylem.

- Transport of water, nutrients, and food throughout the stem.

 

6. Pith:

- Occupies the central portion of the stem and consists of numerous rounded parenchymatous cells with large intercellular spaces.

- Provides structural support and may have a role in storage.

 

 

 

Monocotyledonous stem         

- The anatomy of a monocotyledonous (monocot) stem displays distinct features that are adapted to the plant's structural and functional needs.

 

1. Sclerenchymatous Hypodermis:

- Outermost layer of the stem, composed of sclerenchymatous cells.

- Provides mechanical strength and support to the stem.

 

2. Vascular Bundles:

- Numerous vascular bundles scattered throughout the stem.

- Vascular bundles are conjoint (joined together) and closed (lack cambium for secondary growth).

- Each vascular bundle is surrounded by a sclerenchymatous bundle sheath for protection.

- Central vascular bundles are typically larger than peripheral ones.

- Responsible for the transport of water, nutrients, and food within the stem.

 

3. Parenchymatous Ground Tissue:

- Large, conspicuous ground tissue located between vascular bundles.

- Provides support and storage, especially in the form of parenchymatous cells.

 

4. Phloem Parenchyma and Water-Containing Cavities:

- Phloem Parenchyma: Absent in monocot stems.

- Water-Containing Cavities: Present within the vascular bundles.

- Water-containing cavities play a role in water storage and transport within the vascular bundles.

 

 

Dorsiventral(dicotyledonous)leaf

- A dorsiventral leaf, with its distinct upper (adaxial) and lower (abaxial) surfaces, can be divided into three main parts: epidermis, mesophyll, and vascular system.

 

1. Epidermis:

- Covers both upper and lower leaf surfaces.

- Contains a conspicuous cuticle for protection.

- Typically, the abaxial epidermis has more stomata than the adaxial epidermis; sometimes, the adaxial epidermis lacks stomata.

- Epidermis serves as a protective layer while stomata regulate gas exchange.

 

2. Mesophyll:

- The tissue between the upper and lower epidermis.

- Cell Types:

a.    Palisade Parenchyma:

Situated adaxial and comprises elongated cells arranged vertically and in parallel. Rich in chloroplasts, it is specialized for photosynthesis.

b.    Spongy Parenchyma:

Found below the palisade parenchyma and extending to the lower epidermis. Cells are oval or round and loosely arranged, with numerous air cavities between them. Also involved in photosynthesis.

- Mesophyll houses chloroplasts and carries out photosynthesis, with the palisade parenchyma focusing on light absorption and the spongy parenchyma facilitating gas exchange.

 

3. Vascular System:

- Vascular bundles, which are visible in the veins and midrib of the leaf.

- Vascular bundle size varies according to the size of the veins.

- In dicot leaves with reticulate venation, veins have different thicknesses.

- Bundle Sheath Cells: Vascular bundles are surrounded by a layer of thick-walled bundle sheath cells, which provide support and protection.

 

 

    

Isobilateral(monocotyledonous)leaf

- An isobilateral leaf shares several anatomical characteristics with a dorsiventral leaf but exhibits some notable differences. Such as -

 

1. Stomata Distribution:

- Isobilateral Leaf:

Stomata are present on both the upper (adaxial) and lower (abaxial) surfaces of the epidermis.

 

- Dorsiventral Leaf:

Typically, stomata are more abundant on the lower (abaxial) epidermis, while the upper (adaxial) epidermis may lack stomata.

 

2. Mesophyll Differentiation:

- Isobilateral Leaf: Mesophyll is not differentiated into distinct palisade and spongy parenchyma layers.

 

- Dorsiventral Leaf:

In dorsiventral leaves, the mesophyll is organized into palisade parenchyma (upper layer) and spongy parenchyma (lower layer).

 

Adaptations in Isobilateral Leaves:

1. Bulliform Cells in Grasses:

 - In certain grasses, specific adaxial epidermal cells along the veins transform into large, empty, colorless cells known as bulliform cells.

- Bulliform cells respond to changes in water content. When turgid (filled with water), they cause the leaf surface to expand, aiding in temperature regulation. When flaccid (due to water stress), they induce the leaves to curl inward, reducing water loss.

 

2. Parallel Venation in Monocot Leaves:

- Monocot leaves typically exhibit parallel venation, where veins run roughly parallel to each other.

- Vascular Bundle Sizes:

In vertical leaf sections, vascular bundles in monocot leaves tend to have similar sizes, except for the main veins.