3.1.3(e) - Xerophyte and hydrophyte adaptations

Water availability

An adaptation is a feature that increases the chance that an organism survives and reproduces in its environment. In this lesson the environmental pressure is water availability: some plants live where liquid water is difficult to obtain or where water loss is high, while others live in or on water.

The shared problem is gas exchange. Leaves need carbon dioxide for photosynthesis, and stomata allow gases to move between the leaf air spaces and the atmosphere. However, open stomata also allow water vapour to diffuse out of the leaf. Transpiration is the loss of water vapour from aerial parts of a plant, mainly through stomata.

A strong explanation therefore links three ideas:

Do not write that a plant is adapted only because its habitat is "hot". Heat may increase evaporation, but OCR's specification point is about availability of water. A cactus and marram grass are xerophytes because water is limited or easily lost; a water lily is a hydrophyte because it lives with abundant surrounding water.

Cacti as xerophytes

Cacti are xerophytes found in arid environments where rainfall is infrequent and water can be lost quickly from exposed surfaces. Their adaptations reduce water loss and help store or take up water when it is available.

Many cacti have thick, fleshy stems. These stems contain water-storage tissue, so water taken up after rainfall can be retained for later use. The stem is also the main photosynthetic surface in many cacti, because the leaves are reduced to spines.

Reduced leaves help because leaves have a large surface area and usually contain many stomata. If broad leaves were present in a dry environment, they would provide a large area for evaporation and diffusion of water vapour. Spines have a much smaller surface area, so they reduce transpiration. They can also reduce herbivory, but the key point for this lesson is reduced water loss.

A thick waxy cuticle on the outer surface reduces evaporation through the epidermis. Stomata can be few, sunken or carefully regulated, so less water vapour is lost through open pores. Cacti may also have shallow, spreading roots that absorb water rapidly from near the soil surface after brief rain.

Marram grass leaves

Marram grass is a xerophyte even though it often grows on coastal dunes rather than in a desert. Sand drains quickly, wind removes humid air from leaf surfaces, and salt spray can make water uptake more difficult. The plant's leaf structure reduces water loss from stomata.

The leaf can roll inwards when water is limited. This reduces the surface area directly exposed to moving air and encloses the stomata inside the rolled leaf. The air trapped inside becomes more humid, so the water vapour concentration gradient from the leaf air spaces to the surrounding air is reduced. A shallower gradient means slower diffusion of water vapour out of the leaf.

Marram grass also has stomata sunk in grooves or pits on the inner surface of the rolled leaf. Leaf hairs, or trichomes, project into this inner space. The hairs slow air movement, helping maintain a humid layer around the stomata. A thick waxy cuticle on the outer surface reduces evaporation through the epidermis.