UNIFORM, FATIGUE, AND STAIRCASE EFFECTS 



109 



there be noticed that the maximum contraction is attained in 

 the course of three minutes, after which there is a fatigue- 

 relaxation, which continues up to the eleventh minute. 

 There then occurs a second, though much feebler, response, 

 after which comes a slow and continuous reversal action. 



So-called ' anomalous ' response in Mimosa. — In con- 

 nection with the subject of fatigue, I shall here enter upon 

 the explanation of certain well-known responsive effects in 

 the case of Mimosa which 

 have hitherto been regarded 

 as anomalous. It is generally 

 found that an erect leaf of 

 this plant is sensitive, that is 

 to say, when stimulated it 

 becomes depressed. In this 

 depressed position it is ap- 

 parently insensitive, hence 

 we are apt to assume that the 

 erect posture is one of sensi- 

 tiveness, depression indicating 

 the reverse. It will be found, 

 however, that if a Mimosa leaf 

 be continuously stimulated 

 by successive blows or taps, 

 in the manner of Pfefifer's 

 experiment, the leaf will at 

 first fall. But, though the 

 blows be continued, the 

 petiole will, after a time, 

 return to its normal erect position. In this erect posture, 

 however, further blows prove to have no effect upon it, the 

 leaf being now insensitive. 



I give above a photographic record of this effect in 

 Mimosa (lower record, fig. 59), continuous stimulation in this 

 case having been produced by tetanic electric shocks. It 

 will be noticed that after its responsive fall the leaf returns 

 to the erect position, in spite of the fact that stimulus is 



Fig. 59. Photographic Records of 

 Normal Response of Mimosa to 

 Single Stimulus (upper figure), and 

 to Continuous Stimulation (lower 

 (figure) 



In the latter case the leaf is erected in 

 spite of continuous stimulation. 



