CONJOINT EFFECTS 123 



fluctuate for similar reasons, and also in accordance with the length of 

 the day l . 



SECTION 27. Conjoint Effects (continued). 



The simplest response involves such factors as the mechanical resistance 

 of the curving organ, as well as the stimulatory effects due to the mere 

 progress of the curvature and to the altered geotropic induction due to 

 the changes of position. The statical moment of a leaf alters when it 

 passes from the horizontally expanded position into a vertical one, and 

 this is bound to exercise a certain influence upon the progress of the 

 curvature, as does also the fact that more work is done when a leaf is 

 raised than when it sinks downwards. 



The energy of movement is, however, usually so great that the 

 mechanical factors due to the weight of the leaf are of minor importance or 

 may be negligible 2 . In the case of Mimosa pudica, however, the secondary 

 petioles move forwards at evening 3 , and the increased statical moment 

 of the leaf causes a pronounced sinking of the primary petiole, which only 

 rises above the position during the progress of the night, although darken- 

 ing during the day always causes it to perform an upward photonastic 

 movement 3 . That this evening fall of the primary petiole results from 

 a photonastic reaction coupled with the increased moment exercised when 

 the secondary petioles come more into line with the main one is shown 

 by the fact that it gradually ceases when the change of position of the 

 secondary petioles is mechanically prevented. The evening movement 

 of the primary petiole then corresponds from the commencement with that 

 produced by darkening, as it does in other plants. When the secondary 

 petioles are released, a certain sinking of the main petiole ensues on the 

 following evening, and increases gradually until, after five or ten days, it 

 has reached its original amplitude. The evening fall is, therefore, due 

 to the co-operation of the evening photonastic action with the after-effects 

 of previous response and the mechanical actions resulting from the move- 

 ment of the secondary petioles. As the result of the induced after-effects, 

 the evening fall of the primary petiole only ceases a week or a fortnight 

 after the secondary petioles have been fixed. 



1 On the opening and closing times of flowers and on floral clocks cf. Kerner, Pflanzenleben, 

 1891, Bd. II, p. 211 (Natural History of Plants, 1895, Vol. u, p. 212); Burgerstein, Ueber die 

 nyctitropischen Bewegungen der Perianthien, 1887, p. 39; Oesterreich. Bot. Ztg., 1901, Nr. i. 



3 Cf. Pfeffer, Period. Bewegungen, 1875, p. 144. The mechanically stimulated leaf shown in 

 Fig. 19, p. 6r, will serve also to show the evening position. 



3 Pfeffer, Period. Bewegungen, 1875, p. 73. The normal progress of the daily movements was 

 described in detail by Millardet, Nouv. Recherches sur la pe>iodicite" de la tension, 1869 (reprinted 

 from M^m. de la Soc. d. sci. nat. de Strasbourg, T. vi). 



