512 



PHYSIOLOGY OF GROWTH. 



FIG. 171. Shoot of Atropa Belladonna, 

 the terminal portion of which has curved 

 epinastically. 



in the box in their 

 normal position curve 

 upwards, but when re- 

 versed do not curve afc 

 all. According to H. de 

 Vries, shoots of Prunus 

 avium behave in a similar 

 manner. In these cases 

 the direction of growth 

 is the resultant effect of 

 the simultaneous action 

 of geotropism and hypo- 

 nasty, i.e. the more 

 active growth owing to 

 internal causes of the 

 under side of the shoot. 



We will now make experiments with leaf -stalks and midribs of 

 leaves similar to those we have conducted with shoots. If, e.g., 

 we experiment with growing leaf-stalks of Calla palustris and 

 Petasites, with the main leaf-stalks of the pinnate leaves of Sam- 

 bucus nigra or Juglans regia (always without the lamina), or with 

 midribs of Sambucus leaves, and stick them with their ends in 

 the sand wall of the zinc box, in the normal position or reversed, 

 we shall always find that they are negatively geotropic and more 

 or less strongly epinastic. I tested especially the behaviour of 

 Calla, and of the midribs of Sambucus leaves. In the normal 

 position they curved downwards, in the reversed position strongly 

 upwards. If leaf -stalks or ribs are placed horizontally in the zinc 

 box, on their side, their median plane therefore being disposed 

 horizontally, then negative geotropism must strive to bring 

 about an upward curvature in a vertical plane, while epinasty 

 tends to produce a curvature in a horizontal plane. The resul- 

 tant curvature actually taking place, must therefore be in an 

 oblique plane. 



