Case Study
Passage with linked questions
Case Set 1
Case AnalysisPassage
During a school practical, Meena was asked to prepare a temporary slide of a cross-section of a leaf and observe it under the microscope. She noticed that the upper surface (adaxial) had very few stomata, while the lower surface (abaxial) had numerous stomata. She also observed that the cells just below the adaxial epidermis were elongated and arranged in neat vertical columns, while the cells below them were loosely arranged with large air spaces. The teacher explained that the leaf she had sectioned belonged to a dicotyledonous plant and that the organisation she observed was typical of a dorsiventral leaf. Meena was curious about why the two halves of the leaf were so different in structure.
Question 1: Name the two types of mesophyll cells Meena observed in the dorsiventral leaf and state their positions.
- The elongated, vertically arranged cells she observed just below the adaxial epidermis are called palisade parenchyma; they are placed on the adaxial (upper) side of the mesophyll.
- The loosely arranged, oval or round cells with large air spaces below the palisade layer are called spongy parenchyma; they are located on the abaxial (lower) side, extending to the lower epidermis.
- Together, palisade and spongy parenchyma constitute the mesophyll, which is made up of parenchymatous cells possessing chloroplasts and carrying out photosynthesis.
Question 2: Why does the abaxial epidermis of a dorsiventral leaf bear more stomata than the adaxial epidermis?
- The abaxial (lower) surface is less directly exposed to sunlight and is therefore cooler, reducing the evaporative water loss per open stoma compared to the hotter adaxial surface.
- The adaxial (upper) surface receives more direct solar radiation and is at a higher temperature; having fewer or no stomata here prevents excessive transpirational water loss.
- The spongy parenchyma with large sub-stomatal air cavities lies adjacent to the abaxial epidermis, facilitating efficient CO2 diffusion from abaxial stomata to all mesophyll cells, making this surface functionally ideal for stomatal placement.
Question 3: Explain the functional significance of the structural difference between palisade and spongy parenchyma in relation to photosynthesis and gas exchange.
- Palisade parenchyma cells are elongated, tightly packed, and rich in chloroplasts; their vertical arrangement maximises the number of chloroplasts exposed to incoming light from the adaxial surface, making this zone the primary site of photosynthesis.
- Spongy parenchyma has loosely arranged cells with numerous large intercellular spaces and sub-stomatal cavities that connect directly to the abaxial stomata; these spaces allow rapid diffusion of CO2 from stomata throughout the mesophyll, supplying the photosynthetically active cells above.
- The functional division — palisade for light interception and photosynthesis, spongy parenchyma for gas exchange — ensures that both CO2 supply and light harvesting are simultaneously optimised within the same leaf structure.