316 LIFE MOVEMENTS IN PLANTS 



under direct stimulation is also observed in pulvinated 

 organs. When light acts from above the upper half of 

 the pulvinus undergoes contraction, resulting in erection of 

 the motile leaf or leaflets. As regards the effect of in- 

 direct unilateral stimulus of light on the distal side of the 

 organ, we found that its effect is an enhancement of 

 turgor (p. 281). Hence the positive tropic curvature under 

 light is brought about, as in the case of other forms of 

 stimuli, by the contraction of the proximal, and expansion 

 of the distal sides of the organ. 



Various analogies have been noticed between phototro- 

 pic and geotropic reactions, and it has been supposed 

 that the two phenomena are closely related to each other. 

 This has even led to assumption that there are photo- 

 tropic particles which function like statoliths in geotropic 

 organs. There is, however, certain outstanding difference 

 between the two classes of phenomena. In the case of 

 light, the incident energy is entirely derived from the 

 outside. But in geotropism, the force of gravity by itself 

 is ineffective without the intervention of the weight of 

 cell-contents to exert pressure on the sensitive ectoplasm, 

 and thus induce stimulation. This aspect of the subject 

 will be treated in greater detail in a subsequent chapter. 



POSITIVE PHOTOTROPIC CURVATURE. 



I shall now describe the phototropic effect of unilateral 

 light in pulvinated, and in growing organs. From the 

 explanation that has already been given, it will be under- 

 stood that the side of the organ directly acted on by 

 light undergoes contraction and concavity. 



Tropic curvature of pulvinated organs : Experiment 

 117.— For this experiment I employed the terminal leaflet 



