Question

The following are live components of phloem except (a) Sieve tube (b) Companion cell (c) Phloem parenchyma (d) Phloem fibre (bast fibre) The following are dead components of xylem except

Short answer

The only live components of phloem are the sieve tube, companion cell and phloem parenchyma. Conversely, the only living component of xylem is the xylem parenchyma.

Step-by-step solution

Identify live components of phloem

The phloem consists of four components: sieve tube elements, companion cells, phloem parenchyma and phloem fibres (or bast fibres). Among these, the sieve tube elements, companion cells and phloem parenchyma are all living cells, hence are considered as live components of phloem.

Identify dead components of phloem

The phloem fibres or bast fibres are the only components of phloem that are dead. Thus, option (d) Phloem fibre (bast fibre) is the correct answer for the components of phloem that are not alive or dead component.

Identify dead components of xylem

The xylem is composed of tracheids, vessels, xylem parenchyma and xylem fibres. Among these, tracheids, vessels and xylem fibres are dead, only the xylem parenchyma is living.

Key concepts

Phloem Components

Phloem tissue in plants is crucial for transporting organic nutrients, particularly the products of photosynthesis, such as sugars. To understand phloem thoroughly, it's essential to recognize its main components:

• Sieve Tube Elements: These are elongated living cells that form the conduits for transporting nutrients. They lack certain cellular elements like a nucleus to maximize space for transporting sap.
• Companion Cells: These cells are closely associated with sieve tube elements and play a vital role in helping load and unload sugars into the phloem. They contain a nucleus and all organelles, supporting sieve tube elements' functions.
• Phloem Parenchyma: These are living cells that store and assist in the lateral transport of nutrients. Their role in phloem is subtle yet integral for overall efficiency.
• Phloem Fibres (Bast Fibres): These are the only dead components within the phloem. They provide structural support due to their thick, lignified walls.

Understanding these components helps us grasp how phloem effectively transports food substances throughout the plant.

Xylem Components

The primary function of xylem tissue is to transport water and minerals from the roots throughout the plant. Xylem is made up of different kinds of cells, each with its specific role:

• Tracheids: These are long, tubular cells that help transport water and provide structural support. They are dead when functional, as their walls become reinforced with lignin.
• Vessels: Similar to tracheids but wider and shorter, vessels form continuous tubes that allow efficient water movement. They are also dead cells, designed for maximum flow.
• Xylem Parenchyma: The only living cell in xylem, these store nutrients and facilitate lateral transport of water and minerals. They're vital for the overall functionality of the xylem.
• Xylem Fibres: These are elongated, dead cells that contribute significantly to the mechanical support of the plant.

This divergence in components allows the xylem to specialize in strength and efficient transport of water and nutrients.

Living and Dead Cells in Vascular Tissues

Within vascular tissues, the distinction between living and dead cells is significant and highlights their functional adaptation. In phloem, most components, such as sieve tube elements, companion cells, and phloem parenchyma, are living. Their cellular activities, including energy transport and metabolic exchanges, are essential for the active process of nutrient distribution.
Contrasting this, xylem largely comprises dead cells. Tracheids and vessels, the main water carriers, are dead at maturity. This dead state, akin to a narrow pipe, with just a wall surrounding voids, is optimal for efficient water transport and mechanical support.
However, xylem parenchyma remains living, crucial for storage and lateral transfer tasks. This intricate balance of living and dead cells in vascular tissues showcases nature's design of complex systems for specialized plant functions.