494 PHYSIOLOGY OF GROWTH, 



turns, but for us it is of special importance that the younger of 

 the turns which were formed round the thick support, become 

 steeper in the course of one to two days, and apply themselves 

 closely to the thin support. This is due to the continued growth 

 of the parts in question, and their consequent geotropic elevation. 

 The older of the turns formed round the thick support on the 

 contrary remain comparatively flat ; they undergo no further 

 changes since the growth of the older internodes has already 

 ceased. 



A vigorous pot plant of Phaseolus, which has made a few turns 

 round a support, is placed upside down, some strips of wood being 

 placed across the flower-pot to prevent the soil from falling out. 

 The younger, still vigorously growing parts of the stem soon begin 

 to unwind from the support, and the end of the stem directs itself 

 upwards. This result only becomes intelligible when we reflect 

 that in every transverse zone of the stem still in a state of growth, 

 there is always, owing to the influence of the rotating nutation, a 

 tendency to keep growing in the direction of a left-handed ascend- 

 ing spiral. After the inversion of the bean plant, the concave 

 side of the stem, turned towards the support, must therefore 

 become convex, and this results in the unwinding of the parts of 

 the internodes which are still growing. 



That gravity is really of importance in the causation of rotating 

 nutation and winding, has already been shown in 187. The 

 following experiment teaches the same thing. The flower-pot in 

 which a vigorous bean plant has developed and already made 

 several turns round a support, is fixed on the clinostat, and slowly 

 rotated in the vertical plane. The rotation is made in a direction 

 opposite to that of the normal winding. We now see that the 

 parts of the stem still capable of growth loosen themselves from 

 the support. The youngest turns unwind, and the shoot stretches 

 itself more or less straight. Here the coiled, still growing part of 

 the shoot, on suspension of the action of gravity, behaves on the 

 clinostat exactly like, e.g., a simply geotropically curved shoot. 

 Such a shoot, if capable of growth, straightens under the conditions 

 described ; so also a coiled shoot, whose power of winding is like- 

 wise dependent on the co-operation of geotropism. The straighten- 

 ing is the result of internal growth determinants which are very 

 generally operative when plant structures which exhibit nutations 

 are placed under conditions in which the curvature determinant 

 does not act. This power of plant structures to compensate 



