42 RESEARCHES ON IRRITABILITY OF PLANTS 



not commence immediately ; there is a latent period of 

 •I second. The responsive movement then begins and 

 proceeds for a time with increasing speed, the maximum 

 contraction being attained about 3 seconds after the shock. 

 The pulvinus remains in the contracted position for a short 

 period. After this the recovery is initiated. The rate of 

 recovery at the beginning is relatively rapid, and very 

 slow towards the end. The maximum rate of recovery is 

 •09 mm. per second, in contrast with the maximum rate of 

 contraction, which is 24 mm. per second. The movement 

 of recovery is thus about three hundred times slower than 

 the movement of contraction. The recovery is completed 

 in about 16 minutes. 



A stronger stimulus, generally speaking, requires a 

 longer period for recovery. The influence of season is also 

 a factor to be taken into consideration. Under the physio- 

 logical depression induced by winter, the responsive process 

 is appropriately modified. The excitability of the tissue 

 becomes depressed. An intensity of stimulus which in 

 summer was effective, becomes in winter ineffective. To 

 evoke response much stronger stimulus has to be employed. 

 The latent period is prolonged and the amplitude of response 

 reduced. And lastly, in winter there is, generally speaking, 

 a great prolongation of the period of recovery. In summer, 

 with vigorous specimens, recovery may be practically 

 complete in as short a time as 8 minutes. But owing 

 to sluggishness induced in winter, on the other hand, 

 the recovery may be prolonged to 25 minutes or more. 

 In a severe winter response may even be abolished 

 altogether. 



I have hitherto dealt in some detail with the responsive 

 movement of Mimosa. In contrast with this may be cited 

 other examples in which the excitatory reaction may be 

 either more rapid or extremely sluggish. 



Response of Biophytum. — As an instance of relatively 

 quick reaction I give (fig. 15) a record of response of leaflet 

 of Biophytum, The maximum fall was here attained in the 



