RESPONSIVE ACTION OF PLANT-TISSUES TO LIGHT 57 1 



or to the pulvini of the leaflets of Biophytuni, it is known that 

 a responsive movement, similar to that evoked by any other 

 form of stimulus, is induced. Now, we have seen that the 

 response of such pulvini is given by means of differential 

 longitudinal contraction. If, then, the effect of stimulus of 

 light in this case is to produce a differential longitudinal 

 contraction, we ought to be able to obtain from a radial 

 organ, subjected to stimulus of light from all sides, a respon- 

 sive longitudinal contraction. 



Effect of diffuse stimulation of light on non-growing 

 radial organs. — In order to demonstrate this, I took a radial 

 style o{ Datura, and subjected it to the stimulus of light from 

 all sides, by throwing a beam of sunlight which struck it on 

 one side directly, and on others by reflection from properly 

 inclined mirrors. On exposing the specimen to this stimulus 

 for a period of four minutes, a responsive contraction of seven 

 divisions was induced. On the cessation of light, there was 

 recovery in the further course of a period of nine minutes. I 

 next applied stimulus of light for six minutes, and a respon- 

 sive contraction of eleven divisions was then induced, with 

 a subsequent recovery on the stoppage of light, which was 

 completed in fifteen minutes. We thus see that the effect 

 of stimulus of light in producing responsive contraction is 

 precisely the same as that which is the result of any other 

 form of stimulation ; that a feeble or short-lived stimulus 

 induces a corresponding effect, from which recovery takes 

 place, in a comparatively short time ; and that a strong 

 stimulus — unless it induce fatigue— will bring about a 

 considerable contractile effect, from which the recovery is 

 accomplished in a proportionately longer time. Again, I find 

 that continuous stimulation of light produces a maximum 

 tetanic effect, and that too long-continued action, bringing 

 about fatigue, may induce fatigue-reversal — contraction 

 passing into relaxation— as we found in the case of JMimosa, 

 and of various radial organs which were subjected to too 

 long continued action of stimulus. 



