Plant Water lost / g per hour Cuticle Number of stomata A 0.2 thick few B 1.1 thin many C 0.6 medium medium A student compared the rate of water loss from t...

(a) The plant best adapted to a dry habitat is plant A [1]. Reading the table, it loses the least water, only 0.2 g per hour [1], which is exactly what a dry-habitat plant needs. This fits its structural features: it has a thick cuticle and few stomata, both of which reduce water loss [1]. Plant B (thin cuticle, many stomata, 1.1 g per hour) loses most water and suits a moist habitat.

(b) The loss of water vapour from a plant is called transpiration [1].

(c) The waxy cuticle is a waterproof / impermeable layer on the leaf surface [1], so very little water can evaporate through the epidermis [1]. A thicker layer means an even better seal, forcing water loss to occur only through the stomata.

(d) Water vapour is lost mainly through the stomata [1]; having fewer stomata means fewer pores/openings through which water vapour can diffuse out of the leaf [1], so the overall rate of water loss falls.

(e) A large leaf surface area provides more surface for evaporation / water loss [1], which leads to a higher rate of water loss and a greater risk of drying out in a dry habitat [1]. This is why many desert plants have small or needle-like leaves.

(f) To keep the comparison fair, any two of these variables should be kept constant (1 mark each): temperature; humidity; air movement (wind); light intensity; the area of leaf used [2]. The student already used equal leaf areas, so the environmental conditions are the key variables to control.

(g) A covering of hairs traps a layer of moist / humid air next to the leaf surface [1]. This raises the water-vapour concentration just outside the stomata, lowering the diffusion (concentration) gradient between the leaf and the air, so less water diffuses out [1].