Glossary: Adaptations of Plants in Different Habitats

This chapter's terms in context

Chapter 22 covers three terms describing how a plant's structure suits the particular habitat it lives in: adaptation, xerophyte and hydrophyte. An adaptation, in this biological sense, is any structural, physiological or behavioural feature of an organism that improves its chance of survival and reproduction in its particular environment, and in this chapter the focus is specifically on structural adaptations that help a plant cope with either a shortage or an excess of water in its habitat. This single content standard is unusual in the syllabus for being explicitly comparative: rather than describing one organism's structure in isolation, it requires a candidate to explain the same structural feature, such as leaf size, cuticle thickness or root depth, in terms of the opposite water-related problem it solves in two different types of plant.

A xerophyte is a plant adapted to survive in a habitat with a limited or unreliable water supply, such as a desert or a very sandy, well-drained soil, and its adaptations consistently work to reduce water loss or maximise water uptake: a thick, waxy cuticle reduces evaporation from the leaf surface; small, reduced, needle-shaped or spine-shaped leaves (as in a cactus) reduce the total surface area available for transpiration; sunken stomata, often located in pits below the leaf surface, trap a layer of moist air close to the pore and slow the rate of water vapour loss; and an extensive, deep or widely spreading root system increases the volume of soil from which the plant can extract whatever water is available.

A hydrophyte, in almost complete contrast, is a plant adapted to survive in a habitat with a constant abundance of water, such as a pond or a slow-moving river, where the challenge is not water shortage but rather buoyancy, gas exchange in a water-saturated environment, and structural support without the rigid stem tissue a land plant would need. A hydrophyte typically has large, flat, broad leaves to maximise light capture at the water's surface, air spaces (aerenchyma tissue) throughout its stems, roots and leaves to trap air for buoyancy and to allow oxygen to diffuse to submerged tissues that cannot otherwise access atmospheric oxygen directly, a much-reduced or entirely absent cuticle since water loss is not a threat in this habitat, and a poorly developed or absent root system since water and mineral ions are already available directly through the surrounding water rather than needing to be sought out in soil.

Because a xerophyte and a hydrophyte face essentially opposite water-related challenges, nearly every adaptation in one has a directly contrasting counterpart in the other, thick cuticle versus thin or absent cuticle, reduced leaf surface area versus enlarged leaf surface area, extensive deep roots versus reduced shallow roots, and SPM questions on this chapter are built almost entirely around this contrast, frequently asking a candidate to explain the same structural feature in both plant types side by side rather than describing either type in isolation.

A frequent exam error is explaining a xerophyte's or hydrophyte's adaptation only in terms of what the structure looks like, without completing the explanation with the specific water-related problem that structure solves, stating that a cactus has spines is incomplete without adding that this reduces the leaf surface area available for water loss by transpiration.

A related exam angle asks a candidate to predict which set of adaptations a named unfamiliar plant is likely to show, based only on a brief description of its habitat, such as a plant found growing in a permanently waterlogged swamp or one found on an exposed, sun-baked rocky outcrop, testing whether the underlying reasoning, rather than a memorised list tied to specific named plants such as a cactus or a water lily, has genuinely been understood. A mesophyte, a plant adapted to a habitat with a moderate, reliable water supply such as typical garden soil, is sometimes introduced as a third reference point, showing intermediate features that are neither as extreme as a xerophyte's water-conserving adaptations nor as extreme as a hydrophyte's buoyancy-related ones.