CBSE Class 11 Biology Chapter 8: Anatomy of Flowering Plants NCERT Solutions

Solution: Meristems are specialized regions in plants characterized by actively dividing cells, responsible for plant growth. They are classified into three main types based on their location:

1. Apical Meristem:

Located at the root apex and shoot apex. The shoot apical meristem is found at the tip of the stem and is responsible for the elongation of the stem and the formation of new leaves. The root apical meristem is responsible for the elongation of the root.

2. Intercalary Meristem:

Found at the nodes or bases of leaves, particularly in grasses. This meristem is crucial for the regeneration of grasses after they have been grazed by herbivores. Both apical and intercalary meristems are considered primary meristems as they are present from the early stages of a plant's life.

3. Lateral Meristem:

Appears in the mature tissues of roots and shoots, typically later in the plant's life, hence also called secondary meristems. Examples include fascicular cambium, interfascicular cambium, and cork cambium. These meristems contribute to the increase in the girth of the plant by adding secondary tissues.

Solution: Yes, I agree with the statement. During secondary growth in dicot stems and roots, the outermost layer, the epidermis, is often replaced. To provide protection, a new meristematic tissue called cork cambium, or phellogen, develops in the cortical region. This phellogen is composed of rectangular cells with thin walls. It actively divides and cuts off cells on both its outer and inner sides.

The cells cut off towards the outside differentiate to form cork, also known as phellem. The cells cut off towards the inside differentiate to form the secondary cortex, also called phelloderm.

The cork (phellem) is impermeable to water, which helps prevent excessive water loss, but it does allow for gaseous exchange through small pores called lenticels. The cork cambium (phellogen), cork (phellem), and secondary cortex (phelloderm) collectively form a protective tissue layer known as the periderm.

Solution: Secondary growth in the stems of woody angiosperms involves an increase in the girth of the plant, primarily due to the activity of lateral meristems.

Process:

Initially, the vascular cambium is present as a thin strip between the primary xylem and primary phloem. In dicot stems, the interfascicular cambium develops from the cells of the medullary rays that are adjacent to the vascular bundles. This interfascicular cambium connects with the vascular cambium within the vascular bundles, forming a continuous, complete cambium ring.

The cambium ring is a layer of actively dividing cells that cuts off new cells on both its inner and outer sides. Cells cut off towards the inside differentiate into secondary xylem, and cells cut off towards the outside differentiate into secondary phloem. Typically, the production of secondary xylem is much greater than that of secondary phloem.

As secondary growth progresses, the vascular cambium becomes more active, leading to a significant increase in the amount of secondary xylem (wood) and secondary phloem. The pressure from the developing secondary tissues can cause the outer layers, including the epidermis and cortex, to rupture. To protect the stem, a new meristem, the cork cambium (phellogen), arises in the outer cortex. It produces cork (phellem) on the outside and secondary cortex (phelloderm) on the inside, forming the periderm.

Significance:

Secondary growth is crucial for the survival and development of woody plants. It significantly increases the girth of the stem and root, providing better mechanical support to the plant, allowing it to grow taller and withstand environmental stresses. Furthermore, the increased production of secondary xylem ensures a more efficient transport of water and minerals from the roots to the leaves, supporting a larger canopy and increased photosynthetic activity.

(Note: The provided source text includes placeholders for schematic diagrams which are not rendered here. A typical diagram would illustrate the progression from primary tissues to the formation of the cambium ring, secondary xylem, secondary phloem, and periderm.)