Seeds

A seed holds a tiny plant (the embryo) and a food store, inside a tough coat. Seeds need water, warmth and oxygen to germinate. Plants spread their seeds away from the parent so the seedlings do not compete with it, or with each other, for light, water and space. This is dispersal.

Inside a seed. The tough seed coat protects what is inside: the embryo (a tiny root and a tiny shoot) and its food store. Germination is a seed starting to grow. It needs water, warmth and oxygen, not light or soil: the food store keeps it going. The root grows out first, downwards; then the shoot grows up towards the light. Once its first leaves open, the seedling makes its own food by photosynthesis.

Stretch Test it with cress seeds on cotton wool in four tubes: damp and warm (they germinate), dry (they do not), damp in a fridge (very slowly, if at all), and in boiled, cooled water with a layer of oil on top, which keeps the oxygen out (they do not). Change one condition at a time, and keep the rest the same.

Dispersal by	How	Examples
Wind	Light seeds with wings or parachutes drift away on the breeze	Sycamore, dandelion, thistle
Animals	Juicy fruit is eaten and the seeds pass out in droppings; hooked burrs catch on fur	Blackberry, cherry, burdock
Water	Waterproof fruits and seeds with air inside float away	Coconut, water lily
Bursting pods	A drying pod twists and splits, flinging the seeds out	Gorse, broom, lupin, violet

The sycamore investigation. A sycamore seed has a wing that makes it spin as it falls, like a helicopter blade; the spin slows the fall, so the wind has longer to carry it. To test how wing length affects the fall: make paper seeds with different wing lengths (the independent variable, the one you change); drop each from the same height, with the same mass and the same release (the control variables, kept the same); time each fall (the dependent variable, the one you measure); repeat each drop three times and find the mean; check for an anomalous result, one that does not fit the others, and repeat it or leave it out; draw a line graph of wing length against mean time, and write a conclusion.

* The 1.4 s drop is anomalous: far below the other two. The mean uses the other two drops.

Stretch A stopwatch reads to a hundredth of a second, but your reaction time is about 0.2 s, so small differences between times may not be real: repeats, means and a bigger range of wing lengths all help. Velcro was invented by an engineer who looked at burdock burrs under a microscope after a walk with his dog.