394 Sir J. C. Bose and Mr. G. Das. 



obtained in the stem of Dregea voluhilis, where successive short exposures 

 to thermal rays from a platinum spiral gave positive responses followed by 

 recovery. Continuous exposure gave all the different phases of maximum 

 positive, neutralisation, and reversal into negative (fig. 16), just as under 

 violet and ultra-violet rays from the mercury lamp. 'The reversal into 



Fig. 16. — a, positive responses to short exposure to thermal radiation ; h, positive, neutral, and 

 reversed negative under continued action of radiation. The negative response went off the 

 plate. Successive dots at intervals of 5 seconds. 



negative took place within the comparatively short period of 7 minutes. 

 It will thus be seen that, in inducing phototropic curvature, the heat-rays in 

 sunlight play as important a part as the more refrangible rays of the 

 spectrum. 



17. Photonasty. 



The leaflets of Mimosa pudica, as has been stated before, fold themselves 

 upwards when strongly illuminated either from above or below. Under 

 ■ similar circumstances the leaflets of Biophytum sensitivum and Averrhoa 

 fold downwards. Diffuse stimulation by electric shock brings about upward 

 closure of leaflets of Mimosa, and downward closure of those of Biophytum 

 and Averrhoa. The excitability of the pulvinule is thus greater on the upper 

 side in Mimosa and on the lower side of Biophytum and Averrhoa. 



Expt. 16. — I constructed a very delicate lever, with very little weight, in 

 order to obtain a record of the response of the minute leaflets. Light of 

 moderate intensity from an incandescent lamp was applied on the less 

 excitable side of the pulvinule, that is to say, on the lower side of Mimosa 

 and the upper side of Averrhoa leaflets. The results show that the immediate 



