PHYSIOLOGY 259 



tropism), resulting in a revolving motion of the shoot apex. Stem- 

 climbers occur in very different plant families ; and although an upward 

 growth is essential to their full development, which they do not attain 

 if left on the ground, their stems are not able of themselves to main- 

 tain an erect position. The erect stems of other plants, which often 

 secure their own rigidity only through great expenditure of assimilated 

 material, are' made use of by stem-climbers as supports on which to 

 spread out their assimilatory organs in the free air and light. The utili- 

 sation of a support produced by the assimilatory activity of other plants 

 is a peculiarity they possess in common with other climbers, such as 

 tendril- and root-climbers. Unlike them, however, the stem-climbers ac- 

 complish their purpose, not through the use of lateral clinging organs, but 

 by the capacity of their main stems to twine about a support. The first 

 internodes of young stem-climbers, as a rule, stand erect. By further 

 growth the free end curves energetically to one side, and assumes a 

 diageotropic, more or less oblique or horizontal position. At the same 

 time the inclined apex begins to revolve in a circle either to the right 

 or to the left. This is the movement which it has been customary to 

 speak of as "revolving nutation," but which it is better to term 

 revolving movement. The expression " nutation " is not in this case 

 correct, as by it are understood autonomic movements ; while the 



REVOLVING MOVEMENTS OF STEM-CLIMBERS RESULT FROM THE EX- 

 TERNAL SIMULUS OF GEOTROPISM, which causes a promotion of growth 

 in either the right or left side of the young internodes of the inclined 

 shoot apex. As a result of this, a movement towards the other side 

 is induced. On account of the direct connection of the apex of the .shoot 

 with the lower erect internodes, this revolving movement necessarily 

 gives rise to a similar rotation of the revolving apex on its 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 hand of a watch, and rotates at the 

 same time like the axle to which the hand is attached. Through this 

 rotation of the shoot apex, the part of the stem subjected to the 

 action of the lateral geotropism is constantly changing ; and the revolv- 

 ing 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 re- 

 volving 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 sup- 



