← Responses of plants and animals to their external environment: past papers

AS 91603 · 2023 paper · Abiotic responses

Question 1: Explain Merit

Practises Plant responses

Tulips flower in spring and are pollinated by the wind or by animals. Decades of study have shown how they respond to their abiotic surroundings: they show geotropic responses and both thermonasty and photonasty, and even when cut and standing in a vase they still show geotropism and phototropism.

Explain an advantage to the tulip of negative geotropism and of positive phototropism, and how thermonasty and photonasty each help pollination.

The 2023 paper, page 2 ↗ Marking schedule, page 2 ↗

Work it out

Try it on paper first. Stuck? Take one hint at a time.

  1. 1 The describe answer said negative geotropism keeps the stem upright. How, and why does that help?

    Auxin collects on the lower side of a stem that tips over and makes those cells elongate, so the stem curves back up, even cut and in a vase. Held upright above the leaves, the flower is easy for pollinators to find and its pollen is in moving air.

  2. 2 Why is positive phototropism an advantage?

    Auxin moves to the shaded side, those cells elongate, and the stem bends to the light. The leaves photosynthesise more, making the glucose the plant needs for growth, flowers and seed, and a flower turned to the sun is warm and in view.

  3. 3 Why does it help pollination for the flower to open only when warm?

    Thermonasty, opening in warmth and closing in cold, keeps the flower open exactly when insects and birds are flying, so pollen is carried between tulips; shut in the cold, it is protected from frost and hail.

  4. 4 And to open only in the light?

    Photonasty, opening in light and closing in the dark, keeps it open by day, when pollinators visit and the wind can move dry pollen, and closed at night, so animals that take pollen and nectar without pollinating cannot reach it.
Answer Negative geotropism keeps the tulip upright: auxin collects on the lower side of a tipped stem, those cells elongate, and the stem curves up, even cut in a vase, holding the flower where pollinators find it and the wind reaches its pollen. Positive phototropism bends the stem to the light, as auxin on the shaded side lengthens those cells, so the leaves make more glucose for growth, flowers and seed. Thermonasty, opening when warm and closing when cold, keeps the flower open when pollinators are flying and shuts it against frost and hail. Photonasty, opening in light and closing in dark, keeps it open by day, when pollinators visit and wind moves dry pollen, and shut at night against animals that take pollen and nectar without pollinating. Both make pollination more likely.

What the marker wanted

  • Achieved The describe points: the two definitions, one of the nastic responses, the two tropisms and what each gains, and warmth for pollinators.
  • Merit Explains: the advantage of negative geotropism; the advantage of positive phototropism; the advantage of thermonasty, defined, for the flower opening and pollination; the advantage of photonasty, defined, likewise. Three make M6.

One answer, N0 to E8. Merit points sit above the Achieved ones: two give M5, three M6.

In our words, from NZQA’s 2023 marking schedule.

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Common mistake

Calling a nastic response a growth response. NZQA’s 2023 report lists this among Not Achieved answers: a nastic response is a reversible movement whose direction the stimulus does not set.

How did it go?

Got it moves it further out. Review brings it back tomorrow.