Sir,—The tendril of an English briony, which it sends out to anchor itself to some support, is straight at first. But with growth it develops a spiral. To form a simple spiral, one end must be free to revolve. But in this case both ends are fixed. And so, midway, a kink is formed which is free to revolve. On one side of the kink the spiral is right-handed, on the other side of the kink the spiral is left-handed.
The double spiral gives an elastic anchor cord, and can be pulled out to a straight line without snapping, which would
be inevitable with a single spiral To put it another way, the plant throws out a spiral tendril which is
right-handed part of the length and then changes to a left-handed spiral. It is difficult to see how the sun can decide
this.
Alfred Watkins.
Hereford
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