CBSE Class 11 Biology Chapter 6: Anatomy of Flowering Plants NCERT Solutions

Solution: Meristematic tissues are regions in plants characterized by active cell division, leading to plant growth. These tissues are classified into three main types based on their location and function:

1. Apical Meristem:
   • Location: Found at the tips of roots (root apical meristem) and shoots (shoot apical meristem).
   • Function: Responsible for primary growth, which includes the elongation of roots and stems, and the formation of new leaves and branches.
2. Intercalary Meristem:
   • Location: Occurs at the base of nodes, particularly in grasses and some other monocots.
   • Function: Enables the plant to regrow quickly after being grazed or cut, by adding to the length of the internodes or regenerating lost parts.
3. Lateral Meristem:
   • Location: Found in mature regions of roots and shoots, typically as vascular cambium and cork cambium.
   • Function: Responsible for secondary growth, which increases the diameter or girth of the plant organ. It produces secondary tissues like secondary xylem and secondary phloem (vascular cambium) or cork and secondary cortex (cork cambium).

Both apical and intercalary meristems are considered primary meristems as they are active from the early stages of plant life and contribute to the primary plant body. Lateral meristems are secondary meristems, appearing later in the plant's life to facilitate secondary growth.

Solution: Yes, I agree with the statement. Cork cambium is indeed responsible for forming the cork tissue, which is a vital component of the protective outer layer in plants undergoing secondary growth.

During secondary growth in dicot stems and roots, the outer protective layer, the epidermis, often ruptures. To replace this and provide continuous protection, a new meristematic tissue called the cork cambium, or phellogen, develops in the cortical region. This cork cambium is composed of thin-walled, rectangular cells that divide actively.

The cork cambium cuts off cells on both its outer and inner sides:

• Outer derivatives: The cells cut off towards the periphery differentiate into cork, also known as phellem. Cork cells are dead at maturity, suberized, and impervious to water, providing excellent protection against mechanical injury and desiccation.
• Inner derivatives: The cells cut off towards the inside differentiate into the secondary cortex, also called phelloderm. This tissue is parenchymatous and helps in storage and transport.

The cork cambium, cork (phellem), and secondary cortex (phelloderm) collectively form a protective layer called the periderm. While cork is largely impermeable to water, gaseous exchange can occur through specialized pores called lenticels, which are formed by the breaking of the periderm in certain areas.

Solution: Secondary growth in the stems of woody angiosperms (dicots) is a process that increases the diameter of the stem. It involves the activity of lateral meristems: vascular cambium and cork cambium.

Process of Secondary Growth:

1. Formation of Vascular Cambium Ring: Initially, the vascular cambium is present as a thin layer only between the primary xylem and primary phloem (fascicular cambium). The cells of the medullary rays, located between the vascular bundles, become meristematic and form the interfascicular cambium. The fusion of fascicular and interfascicular cambium results in the formation of a continuous, ring-like vascular cambium.

   Schematic Representation: Imagine a cross-section of a dicot stem. The vascular cambium ring encircles the pith and consists of actively dividing cells.

2. Activity of Vascular Cambium: The vascular cambium cells divide actively, cutting off new cells on both sides.
   • Towards the periphery (outside): New cells differentiate into secondary phloem.
   • Towards the center (inside): New cells differentiate into secondary xylem.
   The amount of secondary xylem produced is generally much greater than that of secondary phloem. The secondary xylem, also known as wood, contains vessels, tracheids, fibres, and parenchyma. The primary phloem gets crushed due to the continuous production of secondary phloem. The vascular cambium is also responsible for forming medullary rays, which are extensions of parenchyma cells that pass through the secondary xylem and phloem, facilitating radial conduction of water and nutrients.
3. Formation of Periderm: As secondary growth progresses and the stem increases in girth, the outer cortical and epidermal layers rupture. A new meristem, the cork cambium (phellogen), arises in the cortex. The cork cambium produces cork (phellem) on the outside and secondary cortex (phelloderm) on the inside, collectively forming the periderm. This replaces the epidermis and provides protection.

   Schematic Representation: The outermost layers of the stem now consist of periderm, followed by secondary phloem, the vascular cambium ring, secondary xylem, and finally the central pith.

Significance of Secondary Growth:

• Increased Girth: It significantly increases the diameter of stems and roots, providing better structural support to the plant, especially in large woody plants.
• Enhanced Transport: The production of extensive secondary xylem (wood) ensures efficient transport of water and minerals from the roots to the leaves, supporting a larger photosynthetic surface area.
• Storage: Secondary tissues, particularly the secondary cortex and heartwood, can serve as storage sites for food reserves and secondary metabolites.
• Protection: The formation of periderm provides a robust protective layer against mechanical injuries, pathogens, and water loss.

The annual rings, formed by the differential activity of vascular cambium in different seasons (more active in spring, less in autumn), are also a result of secondary growth and are used to determine the age of a tree.