MOVEMENTS OF GROWTH. 391 



in water till thoroughly soaked, we observe a somewhat consider- 

 able curvature at the apex of the stem of the embryo which lies 

 between the cotyledons. During the germination of the seed this 

 curvature becomes still more marked, so that the terminal bud 

 emerges from the soil completely nodding over. The convexity 

 of the curvature now, and also later, lies on the side of the 

 epicotyl remote from the cotyledons i.e. on the posterior side of 

 the epicotyl and the nutation takes place in consequence of more 

 rapid growth of this posterior side (see Fig. 130). At a lies the 

 convexity of the curvature. If we grow our Phaseolus plants 

 constantly in darkness, the nutation at the upper end of the seed- 

 ling stem persists for a long time ; it is only in the last stages of 

 germination that the terminal bud directs itself vertically up- 

 wards. This takes place very quickly, on the contrary, when 

 young seedlings e.g. seedlings whose terminal bud has just 

 broken through the soil are exposed to the influence of bright, 

 diffused daylight. The curved epicobyl then speedily and com- 

 pletely straightens. 



If we make an ink-mark on a nutating epicotyl of Phaseolus at 

 the place of greatest curvature (Fig. 130 at a), we find this mark 

 after twenty-four or forty-eight hours, the seedling having natur- 

 ally been kept during that period in darkness, at b. From 

 this it is clear that the originally nutating parts of the epicotyl 

 gradually in the course of growth straighten. The nutation is 

 transmitted to the newly forming parts of the stem. More exact 

 consideration of the marks also frequently shows, however, that 

 the plane of nutation is not always the same. The -nutation must 

 be regarded as quite spontaneous. It takes place even when the 

 seedlings are slowly rotated in the dark round a horizontal axis. 

 For further information see the section on experiments with the 

 clinostat. 



The angle formed by the nutating part of the epicotyl of 

 Phaseolus usually amounts to 180, so that the end bud of the 

 upwardly growing stem is directed vertically downwards. In 

 more accurate observations, however, especially of very actively 

 growing Phaseolus seedlings, we find that the angle does not 

 always remain the same. If one day it is 180, it may on the 

 following be, say, 90; on the third 145. 2 



It appears that most structures exhibit spontaneous nutations. 

 To this class belong also the circumnutations of many seed- 

 lings ; 3 and if we grow, e.g., oat seedlings in the dark, we can 



