40 MOVEMENTS OF CURVATURE 



to keep a flower of Convolvulus closed. In this way, aided by roughnesses 

 on the stem and support, and often by special climbing-hairs as in Humulus 

 and Phaseolus, the fixation is rendered more secure 1 . 



From what has already been said as to the mode of coiling, it is 

 hardly surprising that a good climber should form an extremely regular 

 spiral curve around a cylindrical support, and the slope of the coils is 

 in many cases not appreciably altered by moderate changes in the diameter 

 of the support. With thinner and thinner supports the coils become 

 steeper until a limit is reached which is about that which the coils show 

 when the stem partially straightens after loosely coiling upon a thin 

 support. 



As the result of the circumnutation about a vertical axis, the stem is 

 unable to twine around horizontal, or nearly 'horizontal, supports, and the 

 younger coils may untwine when the support is placed in a horizontal 

 position. Mohl 2 found that a string inclined at an angle of 20 to the 

 horizon was no longer twined round by the stem of Ipomoea purpurea^ 

 and one at an angle of 40 by Phaseolus 3 . 



The thickness of support a twiner can grasp is determined mainly 

 by the length of the circumnutating apex, and when this attains a con- 

 siderable length, as in certain tropical Lianas, twining is possible around 

 supports up to 40 cms. in diameter. Scarlet-runners and Hops may 

 twine around sticks of 8 to 15 cms. in diameter, whereas Convolvulus 

 arvensis and Polygonum convolvulus are usually unable to coil around 

 stems thicker than 3-4 cms. 4 Since the length of the circumnutating apex 

 and other conditions also may vary in the same plant, it is not surprising 

 that de Vries should find plants of Wistaria chinensis twining around 

 supports 1 6 cms. in diameter, whereas Darwin found potted specimens unable 

 to coil around supports of slightly less diameter. 



It depends upon these relationships whether a stem twines around 

 a single or several supports when these are grouped near together. That 

 stems should be able to pass from one support to another, sometimes 

 reascending after hanging downwards, is hardly surprising, or that branches 

 may twine around each other. The form of the support is of some importance, 

 thin stems being able to apply themselves more closely to flat supports 

 than thicker stems can, but the material of the support is only of value 

 from a purely mechanical point of view, the older ideas as to the attraction 



1 Cf. Schenck, I.e., p. 131. a L.c., p. 132. 



3 Voss (Bot. Ztg., 1902, Orig., p. 231) finds that Bowiea volubilis will twine around much- 

 inclined supports if the strongly heliotropic shoots are illuminated on one side. 



* See Mohl, I.e., p. 134; Darwin, I.e., p. 29; Baranetzsky, 1. c., p. 56; Schwendener, I.e., 

 p. 418; Schenck, I.e., p. 121. When woody twiners coil around trees, the latter may be slowly 

 strangulated, the twining stem being often deeply imbedded in the secondary wood. Cf. Schenck, 

 I.e., p. 122. 



