LATENT PERIOD AND REFRACTORY PERIOD 27 1 



in question remained at least approximately constant, in suc- 

 cessive experiments. After this there was commencement of 

 recovery, which was completed — as seen by the return of the 

 spot of light to its exact original position — in the course of 

 five minutes. In order to form an idea of the consistency of 

 results which may be expected from a good specimen, I re- 

 peated this experiment six times in succession, commencing 

 the record each time at the same point on the recording 

 surface as before. The degree to which all these curves 

 coincided with each other in detail is almost incredible. 

 Their rising portions, their flat tops, and their gradual descent 

 during recovery, were all so coincident that the six successive 

 curves appeared as but one. 



In specimens of plants which were not in good condition, 

 fatigue was shown by the gradual diminution in height of 

 successive responses. For accurate standard experiments it 

 is therefore necessary to have specimens which are vigorous. 



We have already seen the prolongation of the latent 

 period which is induced by cold, in the case of Mimosa. 

 I have obtained similar results also in working with Bio- 

 phytum. For example, in a certain experiment, moderate 

 cooling induced a prolongation of two seconds in the latent 

 period. When the plant was allowed to return to the sur- 

 rounding temperature of the room, however, the increase of 

 latent period disappeared. 



Latent period diminished by increased intensity of 

 stimulus. — It has been said before that there are innumer- 

 able gradations between the extreme cases of motile sensi- 

 bility in plants. As regards motility, an extremely sensitive 

 leaflet was that of Mimosa pudica. Somewhat less quickly 

 reacting were those of Biophytum sensitivum, and we had in 

 the leaflets of Philanthus urinaria an instance of extreme 

 sluggishness. 



The latent period of the leaflet of Philanthus, under 

 moderate stimulus, is as long as three minutes, and the maxi- 

 mum contraction is not attained under forty minutes ; but 

 with a stronger stimulus the latent period is reduced to less 



