Problem 153 - Objective Biology for NEET

Question

In root, the xylem is (a) Mesarch (b) Exarch (c) Placed at different places in different plants (d) Endarch

Step-by-Step Solution

Answer

The placement of the xylem in a root is (b) Exarch

Step 1: Understand the Options

Each option represents a different type of xylem placement. Mesarch describes a xylem in which the first-formed primary xylem remains in the center, surrounded by later-forming xylem. Exarch describes a xylem where early-formed xylem is located towards the external side. Endarch describes xylem in which early-formed xylem is centrally located, surrounded by later-formed xylem.

Step 2: Identify the Xylem Placement in Roots

In roots, the xylem is typically arranged in an exarch manner, meaning the first-formed primary xylem elements (protoxylem) is peripheral and the last formed metaxylem is towards the centre.

Step 3: Choose the Correct Answer

Based on the arrangement of xylem in roots, the correct answer is (b) Exarch.

Key Concepts

Xylem placement

The placement of xylem in a plant is pivotal for its water conduction and structural support. Xylem can be arranged in various ways based on plant types and morphological structures.

• Exarch: This is when the first-formed xylem elements (protoxylem) are positioned exteriorly, progressing inward as it develops.
• Endarch: Here, initial xylem forms centrally and newer xylem layers form around it outward.
• Mesarch: Xylem formation starts in the middle and both older and younger xylem are present around it.

In the basic structure of a root, the xylem placement is mostly exarch, which allows the plant to efficiently absorb water and nutrients from the soil. Understanding this placement can explain how plants manage various growth environments and functional adaptations.

Exarch xylem

Exarch xylem is crucial in root development and functions distinctively from other xylem arrangements. It is characterized by protoxylem located near the periphery and newer xylem development inward towards the center. This arrangement is adaptive, aiding roots by:

• Enhancing water uptake via its peripheral protoxylem.
• Ensuring gradual transition from old to new xylem cells which supports root elongation.
• Optimizing nutrient transport, crucial for young roots growing into deeper soil layers.

Through an exarch arrangement, roots can develop stronger, going deeper into the soil, aiding plants that require extensive water and nutrient acquisition.

Primary xylem

Primary xylem is the initial form of xylem tissue found in plants. It develops from the procambium during the primary growth phase, contributing directly to the elongation of the plant. This type of xylem is versioned into two specific forms:

• Protoxylem: These are the first xylem elements to develop and are functional in young, growing regions. They are more narrow, with thinner walls, promoting flexibility.
• Metaxylem: These xylem elements form later, generally larger with thicker walls, contributing to supporting plant rigidity and water transport.

Primary xylem plays an indispensable role in plant growth, as it lays the foundational vascular structure enabling efficient water and nutrient distribution right from the early stages.