In green plants, the main exchange of gases such as oxygen and carbon dioxide with the atmosphere takes place through which of the following structures?

Introduction / Context:
Gas exchange is essential for photosynthesis and respiration in plants. Plants must absorb carbon dioxide for photosynthesis and release oxygen and carbon dioxide at different times. This exchange primarily occurs through specialised microscopic openings on the leaf and stem surfaces. This question tests whether you know the main structures involved in gas exchange in plants.

Given Data / Assumptions:

• We are asked about the main site of gas exchange between plant and atmosphere.
• The options include stomata, palisade tissue, spongy tissue, the midrib of the leaf, and root hairs.
• We assume a typical dicot leaf structure taught in school botany.
• We focus on structures that directly open to the external air.

Concept / Approach:
Stomata are small pores present mainly on the underside of leaves, bordered by guard cells that control their opening and closing. Through these stomatal openings, plants exchange gases with the atmosphere: carbon dioxide enters the leaf, and oxygen and water vapour exit. Palisade tissue and spongy tissue are types of mesophyll cells inside the leaf where photosynthesis occurs, but they are not directly open to the outside air; they communicate internally with intercellular spaces connected to stomata. The midrib contains vascular tissue, and root hairs facilitate water and mineral absorption from the soil, not gaseous exchange with the atmosphere. Therefore, the main structure for external gas exchange is stomata.

Step-by-Step Solution:
Step 1: Recall the basic anatomy of a leaf: epidermis with stomata, mesophyll layers (palisade and spongy), and vascular bundles in the midrib.Step 2: Understand that stomata are openings in the epidermis that directly connect internal leaf spaces with the outside air.Step 3: Palisade tissue contains chloroplast rich cells where photosynthesis takes place, but it is not the structure through which air enters or leaves.Step 4: Spongy tissue has large air spaces that store gases but these spaces communicate with the atmosphere via stomata.Step 5: The midrib simply supports the leaf and contains xylem and phloem, and root hairs function mainly in absorption of water and minerals from soil.Step 6: Therefore, stomata are the correct answer.

Verification / Alternative check:
Botany textbooks consistently describe stomata as the primary structure for gas exchange and transpiration. Diagrams show carbon dioxide entering through stomata and oxygen exiting the same way, with the mesophyll tissues using these gases for photosynthesis and respiration. This repeated emphasis confirms that stomata, not internal tissues or root hairs, are the main route of gas exchange between plants and the external environment.

Why Other Options Are Wrong:
Palisade tissue is an internal photosynthetic layer and does not directly open to the air.Spongy tissue helps in internal gas diffusion within the leaf but relies on stomata for communication with the atmosphere.The midrib is a structural and vascular region, not a gas exchange structure.Root hairs help in absorption of water and minerals from the soil, not in external gas exchange with the atmosphere.

Common Pitfalls:
Some learners confuse the sites of photosynthesis (palisade and spongy tissues) with the structures of gas exchange. Others might assume roots play a major role in gas exchange because they are in contact with soil air. To avoid mistakes, clearly separate functions: stomata are the main gateways for gases, mesophyll tissues are sites of photosynthesis, and roots focus more on water and nutrient uptake.

Final Answer:
The correct answer is Stomata.