TWINING WITHOUT SUPPORTS. 



^n 



directed downwards, the spiral turns uncoil themselves again, and the whole shoot 

 becomes perfectly straight. It is even possible to produce spiral turns in the shoot 

 a second time by again placing the cylinder upright. 



But even when the plants are left to themselves in the open there are frequently 

 formed, as previous observers had already found, free turns which embrace no 

 support. This occurs very often in the manner shown in Fig. 381, when the 

 twining apex grows out beyond the rod up which it had climbed, if at the same time 

 the now free shoot-apex is sufficiently light and rigid to maintain its erect position. If, 

 on the contrary, it is very flexible and grows rapidly in length, it falls into a horizontal 

 or oblique position, and begins to describe 

 circular movements which lead to the for- 

 mation not of close spiral coils but only of 

 open S-shaped curves. 



We now come to the question why 

 the free, horizontally sweeping shoots 

 make no spiral turns. In the first place 

 we have already seen that the apex 

 of a twilling plant coiled round a rod, 

 spontaneously uncoils itself from the rod 

 if the plant is laid in a horizontal position 

 and slowly rotated ; and, as I have said 

 above, even the spiral coils produced in a 

 glass cylinder again become straight if the 

 whole is laid in a horizontal position and 

 slowly rotated. But a long thin sweeping 

 shoot-apex finds itself in the same position ; 

 as its posterior parts make torsions the 

 anterior pliant apical portion is thereby 

 passively rotated, as if it had been fastened 

 to a horizontal rotating axis — a movement 

 which I shall, for reasons which will appear 

 later, designate klinostat-movement. By 

 this means the pendent, freely sweeping 

 and nutating shoot-apex, is placed in the 

 same position as a pot-plant twining round 



a rod, which when placed horizontally and slowly rotated, again uncoils itself from 

 the rod. 



I am indeed convinced that the prevention of the formation of spiral turns in 

 free-growing shoots, simply in consequence of this klinostat-movement, is of great 

 use for the plant; for if every free sweeping shoot were to make spiral turns, it 

 would be impossible for it to seize a support, whereas if this is prevented by 

 means of the klinostat-movement the apex of the shoot maintains a form which 

 enables it to coil round a support immediately it comes in contact with it. 

 Darwin and De Vries had already mentioned the easily demonstrated fact that 

 feebly growing shoots of twining plants make free coils which ■ often have the 

 greatest resemblance to old, free, coiled-up tendrils, and I am of opinion that the 

 [3] XX 



Fin, 381.— Apex of a shoot of Akebia 

 qiiinata which has grown out beyond the 

 support and formed free coils. 



