446 PHYSIOLOGY OF GROWTH. 



curvature in the most actively growing region of the stem, but 

 later when the geotropic erection has come to an end, the greatest 

 curvature is at the base of the growing region, since here gravita- 

 tion continues to act for a time as a stimulus. If we experiment 

 with very rapidly growing thin shoots (e.g. Agrostemma) it is 

 found that at a certain time, the geotropic nutation of the entire 

 shoot however having as yet by no means come to an end, the 

 summit of the shoot is not vertically upright, but owing to geo- 

 tropic after-effect (see p. 443) is carried far beyond ^the vertical. 

 Then, later, under the influence of gravitation the apex directs 

 itself vertically (see Sachs, Flora, 1873). In making the obser- 

 vations it is necessary to sketch at intervals the form of the 

 curvature of the shoot for the time being. 



It is also seen that the curvature at the close of the geotropic 

 nutation is very sharp. In earlier stages of the nutation we see 

 particularly clearly that the curvature does not correspond with 

 the arc of any circle, but on the contrary at a particular place is 

 found a very sharp curvature (with very small radius), from which 

 point, both backwards and forwards, it diminishes. 



1 See especially Sachs in Arbeiten des botan. List, in Wiirzburrj, Bd. 1, and 

 H. de Vries in Landwirthschl. Jahrbiicher, Bd. 9. 



174. The Causes of Geotropic Curvatures. 



The attraction of the earth is, of course, not to be regarded as 

 the source of the force which performs the internal and external 

 work in connection with geotropic curvatures. That force is 

 furnished on the contrary by the plant itself, and gravity, which 

 acts as a stimulus, only liberates it under certain conditions. 



That a large amount of external work is done when negative 

 geotropic curvatures take place is at once clear, if we bear in 

 mind that the shoot-ends, often possessing no inconsiderable 

 weight, are raised in the process. But even in the case of positive 

 geotropic nutations the structures are not simply passively 

 dragged down by their weight, but t themselves actively partici- 

 pate in the movement which takes place. This is clearly shown 

 by the fact that roots performing positive geotropic curvatures 

 will force their way into mercury (see Fig. 145). Into a vessel of 

 about 10 cm. diameter pure mercury is poured to a depth of 3 cm. 



