[Mike Willegal]

Mike Willegal asked the Naked Scientists:

I was wondering if the structure of leaves on trees are designed to help reduce the force of the wind on the tree. This would enable the tree to better resist higher wind speeds.  The reasons why I ask is that many leaves have an airfoil type shape.

I also noticed that the train tunnels in Denver Colorado airport have propellers on the walls which I expect are placed there to enable the train to pass through the tunnel more efficiently.  I wonder if there is a connection?

What do you think?

[LeeE]

The structure of leaves on trees is 'designed' to keep the leaf flat and stop it from curling so it can maximise it's area and intercept as much light and rain as possible.  However, leaves do have a feature that helps reduce wind resistance in high winds - the leaf stem.  The stem allows the leaf to twist and be blown sideways so that it's edge-on to the wind, reducing it's wind resistance.  Because leaves want to have a large surface area, but grow from a relatively small origin i.e. the stem, they must get wider as they grow.  Similarly, and because having leaves that are very large would put more stress on the stem, the leaf has got to limit it's size and eventually become narrower again, so it'll have come back down to another point, or series of points.  I guess it would be possible for leaves to grow with flat ends, as though their tips had been cut off, but I suspect this would require a genetic overhead.  If leaves had an aerofoil shape across or along their surface it would actually increase the force on the leaf and the stresses on the stem as the wind blew across it, not reduce it.

Some trees do use aerodynamics in their seed designs (samaras - from elms, maples and ashes), which allow the seeds to be transported further away from the tree.

I very much doubt that the propellers on the walls in the train tunnels are there to help the train pass through the tunnel more efficiently.  When you think of the less than airtight fit of the trains in the tunnels, the rate of airflow needed to make any difference would need to be enormous, would cause havoc at any stations in the tunnel and would suck in lots of debris at the tunnel entrances.  Generally, the passage of trains through tunnels is enough to maintain sufficient airflow to freshen the air and prevent a build-up of CO2, so I'd guess that they may be there primarily to control the direction of any smoke, during rescue operations, should there ever be a fire in the tunnel.