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Sunday, April 17, 2005

Maple seeds everywhere

Maples have lots of seeds. Each year, I have asked my botany students to guess how many of the thousands of seeds produced by an average single maple, during its lifetime, will grow into a mature tree. They usually give some small number. I point out that the answer is one. If a maple, on the average, produced more than that, even such a small number reaching adulthood as 1.03, say, maple trees would be increasing in number, and would eventually cover the earth's surface. On the other hand, if the average number of surviving adult offspring per tree were less than 1, say 0.984, maples would be declining in numbers, and eventually become extinct.

Every movement, every ethnic group, faces the same challenge. Reproduce at at least the replacement rate, or disappear. In 2001, there were only seven Shakers left.

Apples and oranges aren't the only trees that have fruit. To a botanist, a fruit is a ripened ovary. Fruits are of many shapes and sizes. Most of them contain one or more seeds. The maple tree has a fruit. The official name for this fruit is a samara. (Linked page includes a color photo.) Many people call it a "helicopter," a propeller, or a "whirlybird." Each fruit is mostly a single seed, so people aren't far wrong when they refer to these structures, roughly an inch or so in length, as maple seeds.

A page from NASA (!) mentions the aerodynamic properties of these helicopters.

What happens to all these fruits that don't become adult trees? Many of them are eaten by animals, including squirrels and birds.

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