Anatomy Of Flowering Plants Class 11 Notes

Key Definitions

Anatomy

The study of the internal structure of organisms.

Tissue

A group of similar or dissimilar cells, having a common origin and performing a specific function.

Meristematic Tissue

Tissues containing actively dividing cells, responsible for growth.

Permanent Tissue

Tissues composed of cells that have lost the power of division and are specialized to perform specific functions.

Vascular Bundle

A strand of conductive tissue (xylem and phloem) in plants, typically surrounded by supporting tissue.

Cambium

A lateral meristem responsible for increasing the girth (diameter) of the plant stem and root.

Periderm

The protective tissue that replaces the epidermis in older stems and roots, comprising cork, cork cambium, and secondary cortex.

Annual Rings

Concentric rings of secondary xylem, each representing one year's growth, visible in the cross-section of a woody stem.

Plant Tissues: The Building Blocks

Plants are organized into various tissues, which are groups of cells with a common origin and specific functions. These tissues can be broadly categorized into meristematic tissues and permanent tissues.

Meristematic Tissues: These tissues consist of actively dividing cells, responsible for the continuous growth of the plant. They are typically small, with dense cytoplasm, prominent nuclei, and thin cell walls. Meristems are classified based on their position:

1. Apical Meristems: Found at the tips of roots and shoots, responsible for primary growth (increase in length).
2. Intercalary Meristems: Located at the base of leaves or internodes (e.g., in grasses), responsible for increasing the length of the organ and regenerating parts removed by grazing.
3. Lateral Meristems: Found along the lateral sides of stems and roots, responsible for secondary growth (increase in girth or diameter). Examples include vascular cambium and cork cambium.

Permanent Tissues: These tissues are derived from meristematic tissues but have lost their ability to divide. Their cells are specialized to perform specific functions. Permanent tissues are further divided into:

1. Simple Permanent Tissues: Composed of only one type of cell.

• Parenchyma: The most abundant tissue, typically isodiametric cells with thin cell walls. Functions include storage, photosynthesis (chlorenchyma), and secretion. Found in cortex, pith, mesophyll.
• Collenchyma: Consists of cells with unevenly thickened corners, providing mechanical support to young stems and petioles. Present in dicot stems, absent in monocots and roots.
• Sclerenchyma: Composed of thick-walled, lignified cells, often dead at maturity. Provides mechanical support and protection. Two main types: fibres (elongated) and sclereids (stone cells, irregular shapes).

1. Complex Permanent Tissues: Composed of more than one type of cell working together as a unit to perform a common function.

• Xylem: The principal water-conducting tissue. Composed of four elements: tracheids, vessels, xylem parenchyma, and xylem fibres. Tracheids and vessels are the main conducting elements. Xylem also provides mechanical support.
• Phloem: The principal food-conducting tissue. Composed of four elements: sieve tube elements, companion cells, phloem parenchyma, and phloem fibres. Sieve tube elements, along with companion cells, are responsible for transporting sugars.

Understanding the structure and function of these diverse tissues is fundamental to comprehending the overall plant body plan and its adaptations.

Comparative Anatomy: Monocot vs. Dicot

Mechanism of Secondary Growth

1. — In dicot stems, the vascular cambium originates from the intrafascicular cambium (present between primary xylem and phloem) and interfascicular cambium (formed from medullary rays). These join to form a complete cambial ring.
2. — The cambial ring becomes active and begins to cut off new cells both inwards and outwards. Cells cut off towards the pith mature into secondary xylem, while cells cut off towards the periphery mature into secondary phloem. Secondary xylem is produced more rapidly than secondary phloem.
3. — The activity of the vascular cambium is influenced by seasonal variations. In spring, it is more active, producing springwood (early wood) with wider vessels. In winter, it is less active, producing autumnwood (late wood) with narrower vessels. These form distinct annual rings, which can be used to estimate the age of the tree.
4. — As the stem increases in girth due to secondary growth, the epidermis gets stretched and eventually breaks. To provide protection, another lateral meristem, the cork cambium (phellogen), develops in the outer cortical region.
5. — The cork cambium cuts off cells towards the outside, forming cork (phellem), which are dead, suberized cells. Towards the inside, it cuts off cells forming the secondary cortex (phelloderm), which are living parenchymatous cells. The cork cambium, cork, and secondary cortex together constitute the periderm.
6. — All tissues exterior to the vascular cambium, including secondary phloem, periderm, and remnants of primary tissues, are collectively called bark. At certain regions, the phellogen cuts off parenchymatous cells instead of cork cells, which rupture the epidermis, forming lens-shaped openings called lenticels, facilitating gaseous exchange.

Identifying Plant Tissues

• {"exampleTitle":"Tissue Identification (Function)","exampleBody":"Q: A tissue is composed of living, elongated cells with unevenly thickened walls at the corners, primarily providing mechanical support to a young stem. Identify this tissue.\nA: This description perfectly matches Collenchyma."}
• {"exampleTitle":"Tissue Identification (Structure)","exampleBody":"Q: You observe a cross-section under a microscope showing scattered vascular bundles in a large, undifferentiated ground tissue. Which plant structure are you likely observing?\nA: This arrangement is characteristic of a Monocot Stem."}

Key Points to Remember

• Meristematic tissues are responsible for plant growth, while permanent tissues perform specialized functions.
• Primary growth (length) is due to apical and intercalary meristems; secondary growth (girth) is due to lateral meristems (vascular and cork cambium).
• Xylem transports water and minerals; Phloem transports food (sugars).
• Parenchyma is for storage and photosynthesis; Collenchyma for support in young parts; Sclerenchyma for strong mechanical support.
• Dicot roots have 2-6 radial vascular bundles, while monocot roots have more than 6 (polyarch). Pith is large in monocot roots.
• Dicot stems have vascular bundles in a ring, while monocot stems have scattered bundles.
• Dicot leaves are dorsiventral with differentiated mesophyll (palisade & spongy); monocot leaves are isobilateral with undifferentiated mesophyll.
• Vascular cambium forms secondary xylem and phloem; cork cambium forms cork and secondary cortex (periderm).
• Annual rings indicate the age of a tree and reflect seasonal variations in cambial activity.

Exam Tip: Diagrams & Distinctions

Diagrams are paramount in this chapter! Practice drawing and labeling the transverse sections of dicot root, monocot root, dicot stem, monocot stem, and dorsiventral/isobilateral leaf. Examiners often ask to differentiate between monocot and dicot structures based on diagrams or tabular comparisons. Pay close attention to the arrangement of vascular bundles, the nature of the hypodermis, and the presence/absence of cambium and pith. Memorize key distinguishing features. Questions on the process of secondary growth are also common, requiring a step-by-step explanation. Focus on the origin and activity of vascular and cork cambia.

Practice Questions with Solutions

• Q: Differentiate between springwood and autumnwood. A: Springwood (early wood) is formed during spring, has wider vessels, and lighter color due to more active cambium. Autumnwood (late wood) is formed in winter, has narrower vessels, and is darker, formed due to less active cambium.
• Q: What is the primary function of lenticels? A: Lenticels are lens-shaped openings in the periderm that allow gaseous exchange between the atmosphere and the internal tissues of the stem.
• Q: Which tissue is responsible for providing mechanical support to young dicot stems, and what are its characteristic features? A: Collenchyma provides mechanical support to young dicot stems. Its characteristic features include living cells with unevenly thickened corners of the cell wall, and it lacks intercellular spaces.
• Q: What is a conjoint vascular bundle, and where is it typically found? A: A conjoint vascular bundle is one where xylem and phloem are jointly situated on the same radius. It is typically found in stems, particularly monocot stems where they are also closed (without cambium).

Frequently Asked Questions

Frequently Asked Questions