The correct option is 1, 2 and 3 only.
Explanation
Plants, unlike animals, do not possess specialized respiratory organs or a circulatory system for gas transport. In bulky plant organs (like thick woody stems and roots), gas exchange relies entirely on diffusion. The internal structure of these organs is adapted to ensure that diffusion is sufficient to meet respiratory needs.
Statement-wise Analysis:
- Statement 1 is Correct: In bulky stems and roots, the majority of the volume is occupied by the central cylinder (xylem/heartwood), which consists mostly of dead cells. These cells provide mechanical support but do not respire, thereby reducing the overall oxygen demand of the organ.
- Statement 2 is Correct: The living cells in a woody stem are organized in thin layers located just beneath the bark (cambium, phloem, and living parenchyma). These cells are in contact with the external environment through openings called lenticels, which facilitate direct gas exchange.
- Statement 3 is Correct: In plants, each part (root, stem, leaf) takes care of its own gas exchange needs. Consequently, there is very little long-distance transport of respiratory gases from one part of the plant to another, unlike in animals where blood transports oxygen.
- Statement 4 is Incorrect: Parenchyma cells are characterized by loose packing with a network of interconnected intercellular air spaces. This arrangement is essential to allow gases to diffuse freely through the tissue. If the packing were too tight to allow air spaces, diffusion would be inefficient, and the inner cells would suffocate.
Key Takeaway:
Respiration in bulky plant organs is efficient because living cells are superficial, the interior is largely dead (mechanical tissue), and loose parenchyma tissues with air spaces allow for local gas diffusion.