'How do seeds travel?' (Teacher Support)

This Teacher Support page relates to the student 'How do seeds travel?' Challenge.

The overall challenge

How seeds spread is very important to the survival of native plants and in the control of weeds in native areas, on farms, in horticulture and in gardens. This challenge is split into five lessons and is about the ways plants disperse their seeds, how we can help seeds from native plants to spread, and how we can slow the spread of weed seeds.

Students are expected to progress their learning through the following steps: 1) building a knowledge of how seeds are spread, 2) using evidence to support claims, and 3) how to use their knowledge and science capabilities to address real life challenges, namely, enhancing seed dispersal of native plants and controlling weed species.

Lesson 1

Answers to the 'How do these seeds disperse? sheet

Wheat (an introduced plant): Today, wheat seeds are dispersed intentionally by humans who cultivate wheat for food. When farmers harvest, save, then sow the wheat into their fields, they are dispersing the seed from the parent plants.

Rowan (an introduced plant): Rowan trees produce vast quantities of bright red berries that attract native and introduced birds. Once they are eaten the fruit is digested, but the seeds pass through the digestive system unharmed and are pooped out, along with some 'fertiliser'. This can occur some distance from the parent tree, and as a result, rowans have become a weed species in some of NZ's wild areas.

Thistle (an introduced plant): The fluffy seeds (called 'fairies' by children) are dispersed great distances by the wind. They are also produced in vast numbers, which helps them greatly because the chances of a seed landing in a suitable place for germination is very low.

Sycamore (an introduced plant): Sycamores are large trees. Their seeds flutter slowly downwards in a spinning action much like a helicopter, allowing them to be dispersed by wind gusts, sometimes up to hundreds of metres.

Tōtara (a native plant): The tōtara tree's brightly coloured berry and attached seed is eaten by native birds such as the kereru/wood pigeon, korimako/bellbird, tūī and tauhou/silvereye. Once again, the seeds are pooped out some distance from the parent tree.

Oak (an introduced plant): The ripe seeds (acorns) initially fall to the ground by gravity and may sprout wherever they land. In New Zealand they are also dispersed by animals such as rats (which hoard them in burrows, but some of the seeds are forgotten and eventually sprout) and pigs (which swallow some of them whole and pass them out in their poop). In their native lands of the Northern Hemisphere they are also dispersed by squirrels.

Kareao/Supplejack (a native plant): The brightly coloured berry of this vine is eaten by native birds such as kereru/wood pigeon, korimako/bellbirds, and tūī. Once again, the seeds are pooped out some distance from the parent vine.

Hutiwai/Bidibid (a native plant): The spines of these seeds catch on to bird feathers and the fur of mammals, and eventually fall off some distance from the parent plant.

Pine (an introduced plant): Cones of pine trees split open during fire or extreme heat. The helicopter-type seeds that nestle amongst the cone's scales are flung out and wafted away by wind gusts.

Raupō/bulrush (a native plant): Since this plant often grows in the margins of lakes and swamps, its tiny seeds fall, often in clumps, into water and float somewhere else where they germinate. Individual seeds are also light enough to be carried by the wind.

Pea (an introduced plant): As with wheat, they are dispersed intentionally by humans. They can also be dispersed by mammals such as mice and rats.

Kamu/hook grass (a native plant): These seeds have hooks that catch on to bird feathers, and the fur of mammals, and are perfect for catching onto the hairs of human legs as they walk through the bush. Eventually they are pulled off, or fall off some distance from the parent plant.

Coconut (found in tropical regions of the world): These huge seeds initially fall to the ground by gravity and may sprout wherever they land. They may also fall into the sea, float huge distances on ocean currents and end up germinating on a beach on a completely different island or continent.

Apples (an introduced plant): Apple seeds are dispersed unintentionally by humans, e.g. by throwing away an apple core. They can also be dispersed by mammals such as pigs, and, in their place of origin (Central Asia), by bears. The whole fruit is eaten, but the pips survive the journey through the animal's gut and are pooped out.

Harakeke/NZ flax (a native plant): Like raupō, the plants grow in or near water and the seeds can float in water. They are also light enough to be carried some distance by the wind. (Their fluttering motion slows their fall, giving them a chance to be wafted away by wind gusts.)

Lesson 2

In this lesson students gather evidence to support their claims about how given seeds disperse. In science it's not good enough to look at something, then guess what is happening. We need evidence. This can be gathered by careful and repeated observations, measurements, simulations of the dispersal mechanisms of seeds, testing by trial and error, and full-on experiments (fair tests). Here are some examples:

• Testing if a seed can became caught and transported by fur or feathers.
• Measuring how long it takes for a seed to flutter to the ground.
• Testing if a seed floats.
• Testing if a pet rat will hoard acorns or dried peas.
• Heating pine cones or seed pods in a low heat oven or with a hot air gun to see if they will crack open.
• On a windy day, dropping sycamore seeds from a high 'launch pad'. Compare how far away a seed with its wing cut off travels, with the distance a complete sycamore seed travels.
• Testing if given berries will be eaten by birds when presented to them on a bird feeder.

Discussing and debating their claims should focus on using the evidence they have gathered.

Lesson 3

This lesson reinforces students' ideas about how to gather evidence in support of claims. But this time, the evidence gathering begins in the field when they collect seeds they find themselves. Students observe the 'parent' plants, which should help build an appreciation of the broader strategies that a plant uses for dispersing its seeds.

It is not important to be totally correct about the dispersal mechanisms. Rather it is the process of investigation and the use of evidence to debate and support their claims, that are important.

Lesson 4

Now that students understand how seeds are dispersed, they can put their knowledge to use in a practical sense. Of course it is common to propagate seeds in a nursery, then plant the resulting seedlings to establish native plantings. However, an understanding of seed dispersal should open up other opportunities to proliferate the native plant species we desire. Perhaps this could happen en masse through encouraging the spread of seeds in a native area? Use some of the hints to think about how this might be done. Perhaps students can come up with novel ways of achieving this? Research, ponder, trial, discuss and debate appropriate ideas.

Lesson 5

One of the greatest challenges to establishing and maintaining native plants is preventing the spread and growth of weeds that compete with, and sometimes smother the plants. Gaining an appreciation of how weed seeds can spread should give students an idea of the problems encountered when trying to control them. Is it practical to remove the seed heads of weeds before seed dispersal? If so, in what season do we do this? Do we focus our efforts on weeds that are very prolific producers of seeds? Do we grub the whole weed out, or spay it with herbicide? If so, how selective do the sprays need to be? What are the advantages and disadvantages of each of these methods? Are there any grazing animals, insect herbivores or parasites that could be used to target weeds? Students could research, ponder, trial, discuss and debate appropriate ideas.