490 PHYSIOLOGY OF GROWTH. 



shoots already circumnutating, in that case merely binding the 

 lower parts of the shoot to a stick, so that the end of the shoot to a 

 length of 10-15 cin. is left free and quite vertical. In each case 

 we shall perceive sooner or later, how the free shoot end for a 

 length of a few cm. begins to execute a generally very sharp 

 curvature. This so-called preliminary curvature \_Vorkrnmmung~} 

 is not caused simply by the. weight of the terminal bud, as follows 

 from the fact that it persists if we place the shoot in another posi- 

 tion. 



The following is also important. Shoots capable of rotating 

 nutation are like 'other shoots geotropically irritable in their grow- 

 ing region. If, e.g., we stick shoot ends of twining plants into 

 moist sand as indicated in 173, their negative geotropism is clearly 

 exhibited. The zone of greatest irritability is always situated 

 a few cm., e.g. 10 cm., distant from the apex of the shoot, as can 

 easily be made out by using portions of different lengths. 



These facts were first established in detail, by Baranetzki 1 and 

 Wortmanii. 2 They both showed also that the rotating nutation 

 itself is not, as was before supposed, purely spontaneous, but is 

 the resultant of tw T o forms of movement. The shoots of twin- 

 ing plants are of course capable of spontaneous movements, and 

 in consequence of this they perform the so-called " flank-curva- 

 tures " to which also the above-mentioned preliminary curvature 

 owes its origin. This flank- curvature, to be designated as the hori- 

 zontal component of the rotating movement, is distinguished by 

 the fact that always one flank of the shoot thus, e.g., in left- 

 winding plants the right grows more vigorously than all the 

 rest, and hence, if it alone were operative, would result in a 

 spiral coiling of the shoot ends. With the horizontal component, 

 however, is associated another acting vertically, viz., negative 

 geotropism. As was emphasised, and as is very important, it 

 does not act on all zones of the shoot with equal energy, and leads 

 further to the result that the convexity of the curvature of rotat- 

 ing shoots is always normally on their upper side. Hence is 

 brought about a displacement of the flanks and the commence- 

 ment of circumnutation. (See Wortmann, Bot. Zeitung, 1886, 

 pp. 638-642.) 



We cannot here pursue further Wortmann's theoretical con- 

 siderations, there and elsewhere expressed. Essentially I consider 

 them sound. 



It is now of special importance, with a view to proving that 



