Question

Companion cells are usually seen associated with (a) Fibres (b) Vessels (c) Tracheids (d) Sieve tubes

Short answer

The correct answer to the question is (d) Sieve Tubes.

Step-by-step solution

Understand the Terminologies

Companion cells are specialized parenchyma cells, found in phloem of flowering plants. Fibres, Vessels, and Tracheids are associated with xylem; whereas Sieve Tubes are part of phloem.

Connect Companion Cells with Correct Tissue

Since Phloem and Xylem are both different, and knowing the fact that Companion Cells belong to Phloem, we can eliminate answers (a), (b) and (c). This is because Fibres, Vessels, and Tracheids belong to Xylem.

Confirming the Correct Answer

The remaining option is Sieve Tubes, which are indeed a part of Phloem, just like Companion Cells. Therefore, companion cells are usually seen associated with Sieve Tubes.

Key concepts

Phloem

Phloem is a crucial tissue in plants, responsible for the transport of nutrients. Unlike xylem, which primarily moves water and minerals, phloem carries organic molecules such as sugars. Phloem is composed of several cell types including sieve tubes, companion cells, phloem fibers, and phloem parenchyma.

The main function of phloem is translocation, which means moving the products of photosynthesis throughout the plant. This helps support growth in all parts of the plant. Phloem is a living tissue, meaning its cells are alive at maturity, allowing for dynamic control over nutrient distribution.

• Phloem is part of the plant's vascular system, akin to a highway for nutrients.
• Sieve elements and companion cells are key to its function.
• It mainly transports sap, a sugary liquid that is essential for plant growth.

Sieve Tubes

Sieve tubes are specialized cells within the phloem that facilitate transport of substances. They form the main channels through which the plant sap flows. Sieve tubes are elongated, tubular structures that connect end to end to form tubes or vessels.

Each sieve tube is characterized by pores at the ends, known as sieve plates. These plates allow the movement of nutrients but require companion cells to function optimally. Companion cells help in loading and unloading sugars into the sieve tubes. Although sieve tubes are alive, they lack nuclei and depend on companion cells for managing cellular activities.

• Sieve tubes and companion cells are connected through plasmodesmata.
• Their primary role is to transport photoassimilates, primarily glucose.
• Sieve tubes' functional efficiency is crucial for the plant's overall health.

Xylem

Xylem is another key tissue in plants, tasked with the transportation of water and dissolved minerals from the roots to all parts of the plant, autonomously from phloem. Unlike phloem, xylem is a non-living tissue at maturity. It becomes lignified, providing structural support to the plant.

Xylem is primarily composed of vessels, tracheids, xylem fibers, and xylem parenchyma. Water transport in xylem takes place due to a combination of different processes, such as capillary action and transpiration.

• Xylem helps in maintaining structural integrity while facilitating efficient water transport.
• It works on a principle akin to "drinking straw" mechanisms in plants.
• As it matures, it forms strong wood-like structures within plants.