Page:Darwin - On the movements and habits of climbing plants.djvu/24

 two young upper internodes alone moved. A line painted along the surface of a revolving internode which was at the time convex, became first lateral, then concave, and ultimately convex again. Neither the internodes nor petioles are irritable when rubbed. The movement is in the more usual direction, namely in opposition to the course of the sun; and when the stem has twined round a thin stick, it becomes twisted on its own axis in the same direction. After the young internodes have twined round a stick, their continued growth causes them to slip a little upwards and onwards. If the stick be soon removed, the internodes straighten themselves, and recommence revolving. The extremities of the depending shoots turn upwards, and twine on themselves. In all these respects we have complete identity with phanerogamic twining plants; and the above enumeration may serve as a summary of the leading characteristics of common twining plants.

The power of revolving depends on the general health and vigour of the plant, as has laboriously been shown to be the case by Palm. But the movement of each separate internode is so independent of the others, that cutting off an upper one does not affect the revolutions of a lower one. When, however, Dutrochet cut off two whole shoots of the Hop, and placed them in water, the movement was greatly retarded; for one revolved in 20 h. and the other in 23 h., whereas they ought to have revolved in between 2 h. and 2 h. 30 m. Cut shoots of the Kidney-bean were similarly retarded, but in a less degree. I have repeatedly observed that carrying a plant from the greenhouse to my house, or from one to another part of the greenhouse, always stopped the movement for a time; hence I conclude that naturally exposed plants would not make their revolutions during stormy weather. A decrease in temperature always caused a considerable retardation in the rate of revolution; but Dutrochet (tom. xvii. pp. 994, 996) has given such precise observations on this head with respect to the tendril-bearing Pea that I need say nothing more. When twining plants are placed near a window in a room, the light in some cases has a remarkable power (as was likewise observed by Dutrochet, p. 998, with the Pea) on the revolving movement, but different in degree with different plants: thus Ipomæa jucunda (as maybe seen in the table) revolved in 5 h. 20 m., the semicircle from the light taking 4 h. 30 m., and that towards the light only 1 h.; Lonicera brachypoda revolved, in a reversed direction to the Ipomæa, in 8 h., the semicircle from the light taking 5 h. 23 m., and that to the light only 2 h. 37 m. From the rate of revolution in all