How Can We Make a Pine Cone Open and Close?

In this experiment, we’ll make a pine cone open and close as we wish.

Equipment

• Pine cone

• Bowl

• Water

• Patience

• For the followup experiment: A jar or bottle with wide mouth and more cones

Instructions

The experiment’s instructions can be viewed in the following video:

Explanation

A cone is an organ of coniferous trees, such as pine trees, in which seeds develop. Pine cones open in hot and dry weather and close in moist and cold weather. This behavior has to do with seed development, but its function is essentially unknown. This seems to be in common to all coniferous trees and probably helps the trees’ reproduction.

There are several possible reasons why cones may open in hot and dry weather:

Firstly, before the seed is formed, the cone needs to be pollinated - pollen needs to pollinate the ovules of the flower. The chances of pollination succeeding rises if all cones open in all trees at the same time, in this case determined by the weather, usually the first hot, dry day of spring. Thus, an entire coniferous forest can coordinate its pine opening time, release pollen together and pollinate themselves and each other simultaneously.

Secondly, once the seed is formed inside the cone, if the cone opens in hot, dry weather, it’s likely to fly a longer distance with the wind and spread further away from the tree. This helps the seedling avoid competition over resources with older trees. If cones opened in wet weather, the seeds would get wet and fall to the ground almost immediately.

Thirdly, some of the pine cone scales don’t open until they encounter extreme heat and dryness, such as in forest fires. Immediately after the fire, the cones open and the seeds inside, which were protected from the flames, fall to the soil and allow the forest to regenerate itself. The Aleppo pine has been found to germinate very often in the first winter after a fire.

The opening and closing mechanism of the cone is based on the fact that plant material tends to swell when wet or in a humid environment. Wood is mostly made of cellulose, which is a polymer, or a long chain of smaller molecules, that can absorb water. When wood gets wet, the small water molecules get in between the cellulose chains and push them away from each other, making wood swell. The polymer in diapers absorbs water and turns into gel in much the same way.

The cone scales’ structure allows them to close when drenched in water and open when dry: The lower part of each scale has much more wood than the upper part. The upper part even has a groove in which the seed is found. This structure makes the lower part move to a larger distance than the upper part, closing the cone when wet.

Followup Experiment

The property that makes cones close and open depending on humidity allows us to surprise people who aren’t familiar with it. Let’s take small cones and soak them in water until they’re closed. Then we’ll wipe them and put them in wide-mouthed jars or bottles, as shown in the picture:

Then let the open vessels air dry for a few days, until all the water is evaporated and the cones open up.

When you show these bottles to your friends, they will have no clue how you got the cones in them in the first place!

After the cones are open, you should close the bottles so as to prevent moist air from getting in and making the cones close again.

Dr. Avi Saig
Davidson Institute of Science Education
Weizmann Institute of Science

Article translated from Hebrew by Aviv J. Sharon, M.Sc. student at the Weizmann Institute of Science.

Note for Surfers

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