DIURNAL MOVEMENT OF MIMOSA 579 



Before proceeding further I shall refer briefly to the 

 theory of Millardet iu explanation of the diurnal movement 

 of the leaf of Mimosa. He found that the tension in stems, 

 and presumably its turgor, is increased with rise and de- 

 creased with fall of temperature. The movement of the 

 lateral leaf may, therefore, be due ^to the induced variation 

 of tension in the main axis. Had this been the case the 

 minimum tension would have occuired at the minimum 

 temperature in the morning, and the leaf should have under- 

 gone a maximum fall. The maximum temperature attained 

 in the afternoon should have, on the other hand, brought 

 about the maximum erection. The observed facts are, how- 

 ever, the very opposite to these. Kraus and Millardet also 

 found that light and darkness had great influence on the 

 tension, which increases in darkness and diminishes in light. 

 The tension at dawn may therefore be a resultant of the 

 depressing effect of low temperature opposed by the pro- 

 moting effect of darkness, the latter being the predominant 

 factor. The erect position of Mimosa leaf in the morning 

 may thus be accounted for by the resultant increase of 

 tension of the stem. The explanation of the movements of 

 the leaves is thus to be attributed to the variation of tension 

 in the main axis to which the leaves are attached ; 

 this leads to the conclusion that the leaf-movement should 

 be determined in relation to the plant, and not in rela- 

 tion to the external stimulus. I shall, however, describe 

 a crucial experiment in the course of this paper, which 

 will show that the direction of stimulus of gravity has a 

 determining influence on the periodic movement. The 

 sudden fall of the leaf before evening is again inexpli- 

 cable from the theory of periodic Variation of tension. 



The complexity in the diurnal movement in Mimosa 

 arises from the fact that there are three factors whose fluc- 

 tuating effects are different at different parts of tlie day^ 

 The effect at any particular hour results from the 



