334 LIFE MOVEMENTS IN PLANTS 



the other hand, that the first effect of strong light is a 

 positive curvature, and that under its continuous action the 

 positive effect becomes neutralised (cf. Fig. 121). In the 

 study of photolropic action, the employment of strong 

 light has many advantages, since the period of experiment 

 is, by this means, materially shortened. The continuous 

 record then gives an epitome of the various phases of 

 reaction. 



NEGATIVE PHOTOTROPISM. 



I shall next show the continuity of responsive photo- 

 tropic effects, from the positive curvature to the negative, 

 through the intermediate phase of neutralisation. I have 

 in the preceding paragraph described an experiment where 

 under a given intensity and duration of exposure the 

 excitations of the proximal and distal sides bring about 

 neutralisation, the organ assuming a dia-phototropic posi- 

 tion. If the intensity or duration of the stimulating light 

 be further increased, it is easy to see that while excitation 

 transmitted to the distal side is being increaseil, the excita- 

 tory contraction on the proximal side may, at the same 

 Lime, be decreased owing to fatigue brought on by over- 

 stimulation. 



In connection with this it should be borne in mind 

 that the pulvinus of Mimosa exhibits under continuous 

 stimulation, a fatigue relaxation instead of normal contrac- 

 tion. Similar effects are known to take place in animal 

 muscles. The effect of relatively greater excitation will thus 

 give rise to negative phototropic curvature. The transverse 

 conductivity of organs of diverse plants will necessarily 

 be different. The neutralisation and reversal into negative 

 will thus depend on three factors : the transverse conducti- 

 vity of the organ, the intensity, and duration of stimulus. 



Neutralisation and reversal under increased intensity 

 of light : Experiment 127. — It is advisable to employ thin 

 specimens (in which the transverse distance is small) for 



