(e) adaptations of plants to the availability of water in their environment

The idea

Plants survive where water supply varies, so their structure and physiology must match the habitat. Hydrophytes, living in or on water, gain buoyancy and rapid gas exchange: air spaces (aerenchyma) in stems and leaves, stomata on upper leaf surfaces, and poor root development. Xerophytes face water shortage and reduce loss while storing or conserving water: thick waxy cuticle, sunken stomata in pits, rolled or reduced leaves, dense hairs trapping humid air, and deep or extensive roots. Mesophytes sit between these extremes. The exam asks you to link each named feature to its function in that water regime, not merely to list features.

What earns the marks

• Aerenchyma air spaces in hydrophytes provide buoyancy and a reservoir of gases for submerged tissues.
• Stomata on the upper surface of hydrophyte leaves allow gas exchange with air rather than water.
• Reduced or absent roots in hydrophytes reflect that water is absorbed over the whole surface.
• A thick waxy cuticle in xerophytes reduces cuticular transpiration.
• Sunken stomata in pits trap humid air, reducing the water potential gradient and so transpiration.
• Rolled leaves and dense hairs create a humid microclimate around stomata, lowering water loss.
• Extensive or deep root systems in xerophytes maximise water uptake from soil.
• Each adaptation must be linked to the specific water availability of the habitat, not described in isolation.

Common mistakes and how to put them right

• Listing features without linking each to its function.

  Put it right: State how the feature reduces water loss or aids uptake.

• Assuming all plants reduce stomata.

  Put it right: Hydrophytes often have many stomata on the upper surface, so match the feature to the habitat.

• Confusing sunken stomata with closed stomata.

  Put it right: Sunken stomata remain open but sit in pits that trap humid air.

• Treating aerenchyma as a water-storage tissue.

  Put it right: Its main roles are buoyancy and gas storage.

Exam tips

• Read the stem to identify the habitat (hydrophyte, xerophyte or mesophyte) before choosing an option.
• Reject options that name a real feature but attach the wrong function.
• Where two options seem plausible, check which one correctly links structure to reduced transpiration or improved uptake.