Page:Darwin - On the movements and habits of climbing plants.djvu/97

 separated by short straight portions. This curious and symmetrical structure has been noticed by several botanists, but has not been explained. It occurs without exception with all tendrils which after catching any object contract spirally, but is of course most conspicuous in the longer tendrils; it never occurs with uncaught tendrils; and when this appears to have happened, it will be found that the tendril had originally seized some object and had afterwards been torn free. Commonly all the spires at one end of a caught tendril run in one direction, and all those at the other end in the opposite direction, with a single short straight portion in the middle; but I have seen a tendril with the spires alternately turning five times in opposite directions, with straight portions between them; and M. Léon has seen seven or eight such alternations. Whether the spires turn several limes in opposite directions, or only once, there are as many turns in the one direction as in the other. For instance, I gathered ten long and short caught tendrils of the Bryony, the longest with 33, and the shortest with only 8 spiral turns; and the number of turns in one direction was in every case the same (within one) as in the opposite direction.

The explanation of this curious little fact is not difficult; I will not attempt any geometrical reasoning, but will give only practical illustrations. In doing this, I shall first have to allude to a point which was almost passed over when treating of Twining-plants. If we hold in our left hand a bundle of parallel strings, we can with our right hand turn these round and round, and imitate the revolving movement of a twining plant, and the strings do not become twisted. But if we now at the same time hold a stick in our left hand, in such a position that the strings become