PHYSIOLOGY 283 



longitudinal axis. Through this rotation the torsion, which would 

 otherwise be produced by the revolving movement of the inclined tip 

 of the shoot, is released. (This process will at once become apparent 

 by imitating the movement with a rubber tube.) Thus the apex of 

 a stem-climber sweeps round in a circle like the hands of a watch, 

 and rotates at the same time like the axle to which the hand is 

 attached. By this rotation of the shoot apex, the part of the 

 stem subjected to the action of the lateral geotropism is constantly 

 changing ; and the revolving movement once begun, must continue, as 

 no position of equilibrium can be attained. 



Without the constant and unchanging action of gravitation in determining the 

 direction of the revolving movement, the twining of a shoot continuously about a 

 support is hardly conceivable. It is accordingly not without reason that the 

 revolving movement is a continuous, fixed, geotropic movement, and not an 

 autonomic nutation without definite directive force. Lateral geotropism is a 

 physiological requisite for the climbing, and the existence of stem-climbers as such 

 is dependent upon this peculiar form of geotropism. To this dependence, however, is 

 also due the fact that stem-climbers can only twine about upright or slightly 

 inclined supports. This is, it is true, a limitation to their power of climbing, but 

 one which is not without advantage, for the plants are thus constrained to ascend 

 to freer light and air ( S6 ). 



When an upright support occurs anywhere in the immediate neighbourhood of 

 the apex of a climbing shoot it is sure to be discovered. The apical extremity, the 

 movement of which is but little disturbed by the leaves, which remain for a long 

 time undeveloped, is forced by its lateral geotropism against the support, and by 

 its next revolutions twines around it. If the support be thin, the coils, at first 

 almost horizontal, are only loosely wound about it. Later they become more 

 spiral, and so wind more tightly. This is accomplished by the ultimate predomi- 

 nance of negative geotropism in the coiled portions of the stem, which tends 

 continually to draw out the coils and make the stems upright. This action of 

 negative geotropism is well shown in the case of shoots which have formed free 

 coils without a support. By the resistance offered by the supports to the 

 complete elongation of the spiral stems, the shoots are held firmly in position. In 

 many twining plants the roughness of their surfaces (due to hairs, bristles, hooks, 

 furrows) also assists in preventing the shoots sliding down their supports. The 

 autonomic torsion arising in the older portions of the stems is also of assistance in 

 holding climbing plants, especially those with furrowed stems, tightly wound about 

 their supports. The twining of stem-climbers, as well as the attachment to their 

 supports, is thus due to geotropic processes of growth, and not, as in tendril- 

 climbers, to contact stimuli. By the mechanism of their climbing process twining 

 plants are restricted to moderately stout supports. 



In addition to the autonomic torsions, a torsion from purely mechanical causes 

 is necessarily manifested in the elongation of the coils of a twining stem, which are 

 at first nearly horizontal, so far at least as it is not equalised by the free movement 

 of the apex. 



The direction of the revolving movements, and accordingly also of the windings, 

 of most stem - climbers is constant. The twining stems are for the most part 

 SIXISTHORSE (Convolvulus, Phaseolus, P/tarbitis, etc.). Seen from above, the wind- 

 ings run from the north towards the west, south and east to north again, i.e. just the 



