Problem 215 - Objective Biology for NEET

Question

Assertion: Vascular bundle of dicot stem said to be open Reason: Such vascular bundle contain cambium which possess ability to form secondary xylem and phloem tissue

Step-by-Step Solution

Answer

The assertion is true, and the reason is true as well. Vascular bundles in dicot stems are described as 'open' because they contain cambium, which can form secondary xylem and phloem, allowing for the expansion of the vascular bundle.

Step 1: Understanding the structure of the dicot stem

The stem of a dicotyledonous plant exhibits distinct patches of vascular tissues, which are arranged in a ring. Each patch, or vascular bundle, is conjoint (xylem and phloem are present in the same bundle), collateral (phloem is present towards the outside, and xylem is located on the inside), and open (cambium is present).

Step 2: Understanding the anatomy and physiology of cambium

The cambium is a part of the vascular bundle that consists of thin-walled, longitudinally elongated cells. This layer of cells has the ability to divide, giving rise to xylem towards the inside and phloem towards the outside.

Step 3: Understanding the role of cambium in forming secondary xylem and phloem

Cambium, being a meristematic tissue, possesses the ability to form new cells. Owing to this property, it plays a crucial role in the secondary growth of the plant by forming secondary xylem and phloem.

Step 4: Corelation between the presence of cambium and the vascular bundle being open

Given that cambium has the ability to form secondary xylem and phloem tissue, hence expanding the bundle, it supports the assertion of the vascular bundle being termed open. This open configuration is a characteristic feature of dicot stem.

Key Concepts

Dicot Stem

Dicot stems are distinctive due to their well-organized structure. In a dicotyledonous plant, the stem exhibits vascular tissues arranged systematically in a ring format. These rings are known as vascular bundles and they are crucial for supporting the plant's overall function and growth.

Key Features of Dicot Stems:

• The vascular bundles are conjoint, meaning they contain both xylem and phloem together.
• They are collateral, where the phloem is positioned on the outside and the xylem on the inside.
• Vascular bundles in dicots are open, due to the presence of a layer called cambium.

This ring arrangement allows dicot stems to support more extensive secondary growth, which contributes to the thickening of stems.

Cambium

The cambium is a vital part of the vascular system in plants, particularly in dicots. It is a layer of actively dividing cells that lies between the xylem and phloem. Because of its unique properties, the cambium plays a key role in plant growth.

Characteristics of Cambium:

• Cambium consists of thin-walled, elongated cells that can divide to create more cells.
• Its meristematic nature means these cells are always dividing, contributing to growth.
• As these cells divide, new xylem is formed inside and new phloem outside.

The presence of cambium makes the vascular bundle 'open', as it can produce secondary tissues contributing to increased girth and strength of the stem.

Secondary Xylem and Phloem

Secondary xylem and phloem are produced due to the activity of the cambium. They are crucial for the plant's secondary growth, which is responsible for the thickening of the plant over time.

Role of Secondary Xylem and Phloem:

• Secondary growth occurs after the initial formation of primary tissues and aids in widening stems and roots.
• Secondary xylem, or wood, is what accumulates inside the cambium layer, providing structural support.
• Secondary phloem forms on the outside of the cambium and is essential for transporting nutrients throughout the plant.

This continuous formation is vital for maintaining the plant's health as it matures and grows larger.

Meristematic Tissue

Meristematic tissues are regions in plants that consist of actively dividing cells, crucial for growth. The cambium itself is a type of meristematic tissue.

Characteristics of Meristematic Tissues:

• The cells are small, with thin walls and dense cytoplasm.
• Due to their ability to continuously divide, they are the source of all new cells.
• Different types of meristems (like cambium) initiate growth in different directions: length (primary growth) and girth (secondary growth).

Meristematic tissues like the cambium are essential for both primary growth (elongating the plant) and secondary growth (widening), ensuring the plant can adapt and thrive in its environment.