Let us suppose a caught tendril
to make thirty spiral turns all in the same direction; the inevitable
result would be that it would become twisted thirty times on its own
axis. This twisting would not only require considerable force, but,
as I know by trial, would burst the tendril before the thirty turns
were completed. Such cases never really occur; for, as already
stated, when a tendril has caught a support and is spirally
contracted, there are always as many turns in one direction as in the
other; so that the twisting of the axis in the one direction is
exactly compensated by the twisting in the opposite direction. We
can further see how the tendency is given to make the later formed
coils opposite to those, whether turned to the right or to the left,
which are first made. Take a piece of string, and let it hang down
with the lower end fixed to the floor; then wind the upper end
(holding the string quite loosely) spirally round a perpendicular
pencil, and this will twist the lower part of the string; and after
it has been sufficiently twisted, it will be seen to curve itself
into an open spire, with the curves running in an opposite direction
to those round the pencil, and consequently with a straight piece of
string between the opposed spires.
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