438 LIFE MOVEMENTS IN PLANTS 



organ upwards so as to place it parallel to the incident 

 stimulus. Though the reactions are similar in the two 

 cases, yet the effect of light is termed positive phototro- 

 pism, that of gravity negative geotropism. I would draw 

 the attention of plant-physiologists to the anomalous 

 character of the existing nomenclature. Geotropism of the 

 shoot should, for reasons given above, be termed positive 

 instead of negative, and it is unfortunate that long usage 

 has given currency to terms which are misleading, and 

 which certainly has the effect of obscuring analogous 

 phenomena. Until the existing terminology is revised, it 

 would perhaps be advisable to distinguish the geotropism 

 of the shoot as Z^'nitliotropisni and of the root as Nadiro- 

 t?' op ism. 



RELATION BETWEEN THE DIRECTIVE ANGLE AND 

 GEOTROPIC REACTION. 



When the main axis of the shoot is held vertical, the 

 angle made by the surface of the organ with lines of 

 force of gravity is zero, and there is no geotropic effect. 

 Tie geotropic reaction increases with the directive angle ; 

 theoretically the geotropic effect should vary as the sine 

 of the angle. I shall in the next chapter describe the 

 very accurate electrical method, which I have been able to 

 devise for determination of relative intensities of geotropic 

 action at various angles. Under perfect conditions of 

 symmetry, the intensity of effect is found to vary as the 

 sine of the directive angle. This quantitative relation fully 

 demonstrates that geotropic stimulus acts in a definite 

 direction which coincides with the vertical lines of 

 gravity. 



The conditions of perfect symmetry for study of geo- 

 tronic action at various angles will be fully described in 



