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John Clayton
Most of us in the Midwest at least take trees for granted. They
are so common and seem to grow so easily that we fail to consider
how carefully they have to be designed to survive. The leaves
of trees are especially complex. Not only is their chemistry
that turns sunlight and common chemicals into usable complex compounds
highly sophisticated, but their physical makeup and shape are
also carefully engineered.
Leaves have to be arranged on a tree so that there is an efficient
interception of the sun's rays. If you have ever stood beneath
a tree and looked at its shadow, you know that very little sunlight
is wasted.
In addition to absorbing sunlight and carrying on photosynthesis,
leaves have to be able to endure a great deal of physical abuse.
In severe wind, a leaf has to have minimal drag. If the drag
of leaves is high, a tree will be toppled by even moderate winds.
To have a low drag, the shape is critical. A highly streamlined
object with a gentle rounding upstream and an elongated point
going downstream will experience less than 10% of the drag of
a sphere or a cylinder of equal volume. The complex shapes of
most leaves do not conform to this simple shape we have just described.
Steven Vogel, writing in Natural History magazine (September,
1993, pages 59-62), has found that when exposed to wind, leaves
reconfigure themselves into cones or roll themselves up so that
they are stable in high winds. It is obvious that a rolled-up
leaf or a cone-shaped object is less likely to catch wind than
an open object which can act like a sail. Groups of leaves can
naturally fold into a communal cone, once again minimizing the
drag that they put on the tree.
There are enormous engineering problems involved in the catching
maximum sunlight, having enough volume to carry on sufficient
photosynthesis to supply the needs of the plant, and having a
way to avoid providing sufficient surface area to push over the
tree. The design of leaves that allows all of these characteristics
to be present is incredible. A leaf's stem must resist bending
in an up/down direction in order to catch sunlight. To provide
the rolling up of leaves or the formation of cones, the stem must
permit twisting. This is done by grooves in the stem which are
positioned in such a way to decrease torsional stiffness without
decreasing flexural stiffness.
The common leaf speaks eloquently of the incredible complexity
of all living things. We suggest that the assumption that chance
can explain all of these things takes more faith than does the
admission that intelligent design was the cause.
This article taken from: Does God Exist?, Sept/Oct 1996
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