Question 3: Describe Achieved

The golden dodder (Cuscuta campestris) has no leaves and no roots and lives off other plants. A growth response lets it wind up and around a host, branching into a tangle that can spread from the first host to the plants around it. It fixes itself to the host with a special organ, the haustorium, which grows into the host’s tissues and draws water and nutrients from them. When it flowers is critical to its reproduction. It picks up environmental cues, night length above all, but little is known about what starts it flowering; it is known that its hosts include both short-day and long-day plants, and that it seems to flower when its host does.

Identify and describe the relationship between the dodder and its host and the growth response that lets it wind around the host, and describe short-day and long-day plants.

The 2022 paper, page 8 ↗ Marking schedule, page 5 ↗

Work it out

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

1 What is the relationship between the dodder and its host?
Parasitism: the dodder lives on a plant of a different species (the host) and gains nutrients and water from it, at the host’s expense.
2 What growth response lets it wind around the host?
Positive thigmotropism: a directional growth response to touch, which bends the stem towards and around the support it is touching.
3 How does that bending happen?
By uneven growth: auxin makes the cells on one side of the stem elongate more than those on the other.
4 What are short-day and long-day plants?
A short-day plant flowers when the nights are longer than its critical night length (autumn and winter); a long-day plant when they are shorter (spring and summer).
5 What does living this way give the dodder?
Energy: it takes ready-made food and water from its host without needing roots or leaves.

Answer The dodder is a parasite: it lives on a host plant of a different species and gains its nutrients and water, at the host’s expense. It winds around the host by positive thigmotropism, a directional growth response to touch that bends the stem around its support, caused by auxin making the cells on one side elongate more than the other. A short-day plant flowers when nights are longer than its critical night length, a long-day plant when they are shorter. Living off its host gives the dodder energy without roots or leaves.

What the marker wanted

Achieved Describes, one point each: parasitism, the dodder gaining at the host’s expense; positive thigmotropism, growth that bends the stem around the support it touches; how thigmotropism happens (uneven cell growth due to auxin); a short-day plant; a long-day plant; an advantage to the dodder, such as more energy. Four make A4.

The schedule marks the question as one answer from N0 to E8. Achieved points count first: one or two give N1 or N2, three A3, four A4. Above that, two Merit points give M5 and three M6. In 2022 Question Three, E7 is the first Excellence bullet and E8 both.

In our words, from NZQA’s 2022 marking schedule.

Common mistake

Being unable to name and describe the relationship between the dodder and its host. NZQA’s 2022 report lists this among Not Achieved answers: it is parasitism.

How did it go?

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