GROWTH SUBSTANCES FOR GEOTROPISM 211 



of the main root is brought about by a downward displacement 

 of growth substance induced by the action of gravity. It is 

 probable, also, that the displacement occurs either exclusively or 

 for the most part in the root tip. As shown by Snow (1923), a 

 decapitated root is still able to produce a positively geotropic 

 curvature, provided the tip is again replaced. If the tip is not 

 replaced, the root cannot curve geotropically, even though some 

 growth substance is present in the root stump. 



SUMMARY 



It has become clear that geotropic sensitivity is localized in the 

 tip of the Avena coleoptile and in the tips of the roots of most 

 plants, while in hypocotyls and shoots it is distributed throughout 

 the growing region. The early investigations indicated that a 

 stimulus, probably some chemical substance, was transmitted 

 from the tip back into the growing region. It was shown later 

 (by the agar-block diffusion method) that the growth hormone 

 becomes unequally distributed when the coleoptile, stem, or root 

 is placed in a horizontal position ; more of it is found in the lower 

 than in the upper half. From still other experiments it became 

 clear that the substance in the coleoptile and that in the root are 

 identical. 



The manner in which this growth-regulating hormone is dis- 

 placed unilaterally in an organ subjected to the stimulus of 

 gravity constitutes an important link in the chain of reactions 

 leading to geotropic curvature. When vertically growing organs 

 are placed in a horizontal position, characteristic differences in 

 electrical potential occur; the lower side becomes electropositive 

 to the upper side. Preliminary investigations indicate that this 

 geoelectric effect is independent of the geotropic sensitivity of the 

 organ and the conditions under which it lives. A possible origin 

 of geoelectric potentials has been suggested by experiments with 

 suitable membrane models in which the force of gravity can be 

 shown to have an influence upon diffusion potentials, Geo- 

 electric effects are independent of living processes, whereas 

 photoelectric potentials arise only in living tissues; this must 

 mean that there is a marked difference in their origins. The 

 translocation of growth substance in plants normally takes place 

 toward any region that is electropositive relative to other regions. 

 Artificially applied electrical potentials induce electrotropic 



