PHOTOTROPISM AND GEOTROPISM 289 



Light acting vertically from above induces a concavity 

 of the excited upper side, in consequence of which the organ 

 moves, as it were, to meet the stimulus. The effect of 

 geotropic stimulation is precisely similar. In fig. 169 the 

 arrow represents the direction of stimulus w^hich may be 

 rays of light or vertical lines of force of gravity. 



Analogy between Phototropic and Geotropic 



Reactions 



In geotropic curvature the direction of geotropic stimulus 

 may, for all practical purposes, be regarded as coinciding 

 with the vertical lines of gravity. The analogy between 

 the effects of light and of gravity is very close ; ^ in both 

 the induced curvature is such that the organ moves so as 

 to meet the stimulus. This will be made still more evident 

 in the investigations on torsional geotropic response 

 described in a subsequent chapter. The tropic curve under 

 geotropic is similar to that under photic stimulation. The 

 tropic reaction under the stimulus both of light and of 

 gravity increases similarly with the directive angle. These 

 real analogies are unfortunately obscured by the use of 

 arbitrary terminology in the description of the geotropic 

 curvature of the shoot. The record in fig. 170 gives the 

 response to vertical stimulation by light and by gravity of 

 a horizontally laid bud of Crinum. In both the upper side 

 undergoes contraction and the movement of response carries 

 the 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 phototropism, 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 have the effect 



1 An exception to this will be found on p. 373, with an explanation. 



u 



