(a) Fig. 2.1 shows a vertical section through a leaf. Fig. 2.1 (i) Name the structures labelled P, Q and R. [3] (ii) Describe how the leaf is adapted to red...

Labelled answer diagram:

(a)(i) The structures labelled in the leaf cross-section are:

• P = Waxy cuticle [1] - a waterproof layer secreted by the epidermis that covers the outer surface of the leaf.
• Q = Palisade mesophyll cell [1] - the tightly packed, column-shaped cells just below the upper epidermis, rich in chloroplasts for photosynthesis.
• R = Stoma (stomatal pore) [1] - the opening between two guard cells on the lower epidermis through which gas exchange and water vapour loss occur.

(a)(ii) Adaptations of the leaf to reduce excessive water loss (any three):

1. The waxy cuticle on the upper (and sometimes lower) surface is waterproof, reducing evaporation directly through the epidermis. [1]
2. The stomata can close when conditions are dry - the guard cells lose turgor and the pore narrows, limiting water vapour loss. [1]
3. Some leaves have additional features such as rolled leaf margins or surface hairs (trichomes) that trap a layer of humid air close to the leaf surface, reducing the diffusion gradient. [1]

(b) Most transpiration occurs through the lower surface of the leaf because the majority of stomata are located on the lower epidermis. [1] Stomata are the main exit points for water vapour, so the surface with more stomata loses more water. Having fewer stomata on the upper surface (which faces direct sunlight) helps reduce overall water loss from the most exposed face. [1]

(c)(i) The seedlings wilted after transplanting because the root system was damaged during the uprooting and replanting process, reducing the plant's ability to absorb water. [1] At the same time, moving from a shaded nursery to an open field exposed the leaves to direct sunlight and higher temperatures, which dramatically increased the transpiration rate. Water loss exceeded water uptake, so the cells lost turgor and the seedlings wilted. [1]

(c)(ii) One method the farmer could use to reduce wilting is to provide temporary shade (e.g. using shade cloth or leafy branches) over the newly transplanted seedlings. [1] Other acceptable methods include: watering the seedlings immediately after transplanting, transplanting during the evening or on a cloudy day, or removing some leaves to reduce the transpiration surface area.