Variation and Selection
Senior 3 · Biology
Variation and Selection
Lesson 5 of 5
25 min
Pull up a bean plant and leave it on a sunny bench: within an hour it wilts. A candelabra tree on a rocky hill in dry bushland stays green through months without rain. Take a water lily out of its pond and its broad leaves wilt and shrivel quickly, yet it grows happily with its roots in mud that has almost no oxygen, where a bean plant would drown.
Each of these plants is suited to its own supply of water. In Senior 2 you sorted plants into mesophytes, xerophytes and hydrophytes by their habitat. This lesson looks closely at the structures, explains how each one works, and asks how such structures came about.
Definition
Adaptive feature: an inherited feature that helps an organism to survive and reproduce in its environment.
Xerophyte: a plant adapted to live where water is scarce, such as deserts, dry bushland, rocky ground or sand dunes. Examples: cacti, candelabra trees (Euphorbia), aloes, marram grass.
Hydrophyte: a plant adapted to live in water or in waterlogged soil. Examples: water lilies, water hyacinth, papyrus, pondweeds.
Aerenchyma: plant tissue with large air spaces, found in many hydrophytes.
Worked example
Worked example 1: an experiment on water loss. 20.0 g of leaves was cut from each of three plants and left on a bench for 24 hours, then weighed again (illustrative data): aloe (a xerophyte with fleshy leaves) 18.6 g; bean (a mesophyte) 9.2 g; water lily (a hydrophyte) 4.1 g. Calculate the percentage of mass lost by each and explain the results.
Solution
Fair test: same starting mass, same time, same place (light, temperature, wind). To be safer, repeat with several samples and take the mean.
Every xerophyte feature does one of four jobs: it cuts water loss, it gathers water, it stores water, or it survives drying out. Figure 1 shows several water-saving features in one leaf.
Figure 1. Transverse section of a rolled leaf of marram grass, a xerophyte of dry sand dunes (simplified drawing).
| Feature | How it helps | Example |
|---|---|---|
| Thick waxy cuticle | waterproof layer: less evaporation from the surface | aloe, cactus, marram grass |
| Fewer, sunken stomata; hairs | moist, still air is trapped next to the stomata, so the water vapour gradient is smaller and diffusion is slower | marram grass, oleander |
| Rolled leaves | stomata are enclosed in humid air, out of the wind | marram grass |
| Leaves reduced to spines; green stems | much smaller surface area for transpiration; spines also deter grazing animals | cacti, many euphorbias |
| Fleshy (succulent) stems or leaves | store water for dry periods | aloe, cactus, candelabra tree |
| Stomata open at night, closed by day | carbon dioxide is taken in when it is cool and humid, so far less water is lost | cacti, pineapple |
| Deep roots, or shallow wide roots | reach water deep underground, or catch light rain over a wide area | acacias (deep); cacti (wide) |
For a water plant, water is never short. Its problems are different: getting enough oxygen and carbon dioxide (gases diffuse about 10,000 times more slowly through water than through air), staying near the light, and not being torn by moving water. Figure 2 shows how a floating leaf meets them.
Figure 2. Transverse section of the floating leaf of a water lily, a hydrophyte (simplified drawing).
| Feature | How it helps |
|---|---|
| Large air spaces (aerenchyma) in leaves, stems and roots | make the plant float near the light, and let oxygen pass down to roots in mud that has little oxygen |
| Stomata on the upper surface of floating leaves; none on submerged leaves | gas exchange happens with the air; submerged leaves absorb gases through their thin surfaces |
| Thin or no cuticle | no need to save water; thin surfaces let gases and minerals pass in |
| Little xylem and supporting tissue | the water supports the plant, and water is all around |
| Thin, finely divided submerged leaves; flexible stems | large surface for absorbing gases and light; bend with the current instead of breaking |
| Swollen, spongy leaf stalks full of air | act as floats (water hyacinth) |
Worked example
Worked example 2: a leaf from an unknown habitat. A pupil examines a leaf section and finds: a very thin cuticle; stomata on the upper epidermis only; huge air spaces in the mesophyll; very little xylem. (a) What kind of plant is it? (b) Explain how two of the features suit it to its habitat.
Solution
These features are the result of natural selection (Lesson 3). In a dry place, any plant whose genes gave it a slightly thicker cuticle or fewer stomata lost less water, survived droughts better and left more seeds, so over many generations those alleles became common. Strong evidence comes from unrelated plants that look alike: the cacti of the Americas and the cactus-like euphorbias of Africa belong to different families, yet both have spines, fleshy green stems and few stomata. Similar environments selected similar features in different families.
Warning
Explain, don't just list. "Sunken stomata" earns a mark only with its reason: moist air is trapped above the stomata, the diffusion gradient of water vapour is smaller, so transpiration is slower. And do not write that a plant "grew spines because it needed to save water". The spines arose by chance variation and were selected over many generations.
How A Cactus Survives With So Little Water: Xerophytes Explained, from the Museum of Science. Watch for the features of a cactus that cut water loss and store water, and match them with the table above.
Tip
Four jobs of a xerophyte: Cut, Collect, Store, Survive. Cut water loss (cuticle, sunken stomata, hairs, rolled leaves, spines); Collect water (deep or wide roots); Store water (succulent tissue); Survive drying (seeds or bulbs that wait for rain). For a hydrophyte, think Float and Breathe: air spaces, and stomata where the air is.
Practice: xerophytes and hydrophytes
Key points
Further reading
Sources and credits
Written by Ganzaa: the text, Figures 1 and 2, the tables, the worked examples (illustrative data) and the practice questions.
