Page:Darwin - On the movements and habits of climbing plants.djvu/94

 the extremity of a free tendril were to contract quite regularly, it would roll itself up into a flat helix, as occurs with the Cardiospermum; but if it were to contract in the least on one side, or if the basal portion were first to contract (as does occur), the long free extremity could not be rolled up within the basal part, or if the tip were held during the contraction, as when a tendril has caught some object,—in all these cases the inevitable result would be the formation not of a helix, but of a spire, such as free and caught tendrils form in the act of contraction.

Tendrils of many kinds of plants, if they catch nothing, contract after an interval of several days or weeks into a close spire; but in these cases the movement takes place after the tendril has lost its revolving power and has partly or wholly lost its sensibility, and hangs downwards; this, as we shall presently see, is a quite useless movement. The spiral contraction of unattached tendrils is a much slower process than that of attached tendrils: young tendrils which have caught a support and are spirally contracted may be constantly seen on the same stem with much older tendrils, unattached and uncontracted. In the Echinocystis I have seen a tendril with the two lateral branches clasped to twigs and contracted into beautiful spires, whilst the main branch which had caught nothing remained for many days afterwards uncontracted. In this plant I once observed a main branch after it had caught a stick become spirally flexuous in 7 h., and spirally contracted in 18 h. Generally the tendrils of the Echinocystis begin to contract in from 12 h. to 24 h. after catching something; whilst its unattached tendrils do not begin to contract until two or three or even more days have elapsed after the revolving movement has ceased. I will give one other case: a full-grown tendril of Passiflora quadrangularis which had caught a stick began in 8h. to contract, and in 21 h. several spires were formed; a younger tendril, only two-thirds grown, showed the first trace of contraction in two days after clasping a stick, and in two additional days had formed several spires; hence, apparently, the contraction does not begin in a tendril until it is grown to nearly its full length. Another young tendril of about the same age and length as the last did not catch any object; it acquired its full length in four days; in six additional days it first became flexuous, and in two more days had formed one complete spire. This first spire was formed towards the basal end of the tendril, and the contraction steadily but slowly progressed towards the apex; but the whole was not closely wound up until 21 days had