181 



The nutations are now considered to be movements wliicli take 

 place witliout any stimulations, but tlieir explanation is still 

 wanting. As tliey depend, however, entirely upon longitudinal growth, 

 tliey will be considered here to be induced by the stimulus of 

 gi'avitation. 



The least common case of nutation is seen in the peduncles of 

 Allium I'orrum which first hang over to one side, then straighten 

 out and afterwards bend over lo the other side, and soon. A slight 

 displacement of the static apparatus might induce this movement; 

 if, for instance, the middle-field lies against the basal transverse 

 wall, the stalk, as we have seen, will assume a vertical position; 

 should it then move slightly to one side, the stalk, in its efilbrt to 

 find the new equilibrium, would have to bend over to the same 

 side; if the apparatus then n-ioves back across the transver.se wall 

 and then shifts slightly to the opposite side the stalk would become 

 straight and then also have to bend to that side, and so on. This 

 autonomous shifting of the apparatus lo and fro across the basal 

 transverse wall would thus be sufficient to cause indirectly the 

 "swinging nutation". 



Much more frequent is the "rotating nutation", in which the ti|) 

 of the stem moves as if over a conical surface; it may very well 

 be imagined that this movement is brought about owing to the 

 apparatus, as in the preceding instance, lying somewhat to the side 

 of the tran.sver.se wall bnt is now pushed round in a circle, as it 

 were, though in such a way that the middle-field remains always 

 at the same distance from the centre of the transverse wall. The 

 stem would then again have to follow Ihe whole movement, always 

 making the same angle with the perpendicular. The more the 

 apparatus shifts, and keeps aside from the transverse wall during 

 the nutation, the greater will be tiie angle at the apex of the cone 

 described by the tip of the stem. 



The twining movement was regarded by Sachs as being intimately 

 connected with the rotating nutation, also because at that moment 

 ill both the influence of gravitation seemed to be excluded. Later on, 

 however, it was demonstrated by Noi.i, that in the twining the 

 effect of this stimulus showed itself as "lateral geotropism" where- 

 by the gravitation stimulus brings forth ihe lateral movement of 

 the apex by causing a difference in growth between the two 

 opposite lateral sides of the stem. 



This lateral geotropism thus causes the apex of the stem to swing 

 round, with the tip in a more or less horizontal position, while at 

 the same time the tip twists round its own axis in the opposite 



