Clever Cones

Pine cones open and close without a motor.

Pine trees behave just like any good parent who tries to help their children succeed in life. One way they is by giving each seed (hidden inside the cones) a wing that allows it to travel long distances from its mother tree. It is advantageous for the seeds to get as far as possible from the mother tree so that it does not have to compete for nutrients and sunlight. It is necessary for the seeds to be released from the cone to fly at the right time - the seeds must already be ripe and it must not be rainy either, because dry seeds fly farther in the wind than if they are wet and heavy. Pine trees were able to solve this problem reliably and elegantly. Let's check it out!

This activity starts with closed pine cones. You might be able to find these in a local woodland. If they are open, place them in a bowl of water at least 2 hours in advance to close.

1. Ask pupils to examine the closed pine cones, and to think what conditions will enable them to open up (warmth and dry weather).
2. Invite pupils to select appropriate locations in the classroom (and outside if you wish) which will encourage the cones to open up (somewhere warm and in sunlight). Make a note of the locations and perhaps ask pupils to take a picture of their closed cone.
3. It can take two days or more for the cones to open, so you will need to ask pupils to check their cones daily. Again, they could take a picture of monitor the changes.
4. Ask pupils to observe the gradual opening of the scales of the cones and the appearance of seeds in the crevices.
5. Once opened, invite pupils to take one of the winged seeds from a pine cone and examine it carefully. Find an elevated place where pupils can safely release their seeds and observe how they fly. You can also test how seeds which are dry fly compared with wet ones.

Other options:

Place the closed cones in different places: on the heater, in a dark place, on the outdoor windowsill. Observe the different speed of opening.

If you do not have several days for continuous observation, it is possible to observe the gradual closing of an open cone. Simply place the open cone in a glass of water. Complete closure of the scales takes about 2 hours.

How do trees manage to purposefully open and close their cones to release the seeds at the right moment, when the cones contain no muscles or nerves and are made up of dead cells when mature?    

Simple explanation

When the cone is wet, the scales lay flat against the cone. When the cone dries out, the scales bend backward, and the cone opens. This works well, because the seeds are protected inside the cone until a dry day, which is good weather for seeds to fly.

More detailed explanation

The trick lies in the design of the layers of pine cone scales (see diagram below). Each scale is made of two layers that differ from each other in their ability to absorb water. One layer absorbs water more easily and thus swells more. Conversely, the second layer of the scale swells much less. These different layers are firmly connected together, so during rain, the one that swells more (therefore lengthens more) bends the other. After drying, the tension disappears and the cone opens.

The same principle of changing shape according to changing environmental conditions can be explored with a tape made of two layers of metal with different thermal expansion properties. A so-called bimetallic tape bends according to the changing temperature. This is sometimes how a thermostat indicator works.

Follow-up activity

Provide each pupil a strip of paper on which they stick adhesive tape along its entire length (note - using a slightly thicker paper works best e.g. art paper). They then wet it and observe how it changes shape. Which layer is less absorbent and which more? How would the test strip be oriented on the cone?

Today, we can make a lot of things that automatically respond to changing conditions, like a light that only turns on when it's dark. However, most such inventions require complex sensors, which need a constant supply of energy for their operation. With pine cones, the entire process of opening and closing is spontaneous and does not need any energy supply or systematic management. Instead, the material design of the pine cone interacts with the environment to do everything it needs to do. This could be a great inspiration for engineers to design mechanisms that can move and/or change shape without needing energy input. Such innovations can save energy or other resources due to their efficiency and simplicity, which can be a great relief for the environment.

Imagine new building materials which might respond to changing humidity. In dry, warm midday air, self-adjusting blinds, for example, might bend to create shade and keep houses cool. At night and through humid early mornings, they would relax and open up again. Another possible use is in solar panels that track the sun through the use of bimetallic strips.

Another excellent application, which is already being used, is for clothing that changes its permeability to airflow depending on the humidity. When wet, its permeability increases and when dry it closes again (find out more).

Some STEAM opportunities include:

• Asking questions and making observations.
• Carrying out simple tests.
• Making predictions.
• Using scientific evidence to answer a question.
• Apply learning to real world problems.
• Observing and raising questions about how different species adapt.
• Analyse advantage and disadvantage of different adaptations/behaviours.

Learn more about how pine cones open and close in response to weather (find out more).

Try this more detailed activity to measure the effect of moisture and temperature on pine cones (find out more).