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16-12-2003, 04:02 AM

In article ,
(Phred) writes:
| In article ,
| "P van Rijckevorsel" wrote:
| Keith Michaels schreef
| A snail shell forms an exponential spiral because the direction of
| new growth is proportional to all the growth that has come before.
| If the growth direction is offset on the Z axis then it forms a helix,
| or coil. Starting the helix in one direction (left or right) is
| reinforced by subsequent growth. There is probably a predisposition
| to handedness based on unit shape, and a randomizing component like
| environment (temperature etc.) or a mutation that gets things going
| in one direction or the other.
|
| + + +
| Reminds me of an experiment they did to estimate the proportion of left-
| versus right-handed shells in sea snail shells. This involved a lot of
| manufactured plastic snail shells (sponsored by Shell) which were dumped in
| the sea. At various points along the coast they counted left and
| right-handed shells. It proved that watercurrents performed an advance sort
| and deposited different shells at different points along the coast. You
| could "prove" any proportion of left- versus right-handed shells by
| selecting a collecting spot.

Now wait a minute, if I dump a bunch of conical helixes into the ocean
I would expect them to orient with the pointy end into the wavefronts,
the lefties rotating left and the righties rotating right, and the final
distribution being hourglass shaped with each lobe having a preponderance
of one type or the other. But this has nothing to do with reality; if
you start with a random distribution over a large area the tendency of
one individual to move in one direction is compensated by other
individuals which backfill from nearby areas, assuming new individuals
of both orientations are being born randomly in all areas.

This of course has nothing to do with botany, unless you want to talk
about pea vines and strangler figs.