io8 MOVEMENTS OF CURVATURE 



intensity. If the movement of the leaflets causes the pulvini to be shaded 

 it may cease when a certain inclination is reached, which sometimes gives the 

 appearance of a heliotropic curvature. In addition, the old leaves of 

 Bauhinia (Fig. 31) are unable to fold together as completely as do the 

 young leaves, owing to the increased rigidity of the tissues T .] 



In any case sufficiently strong diffuse illumination produces a sinking 

 of the leaflets of Oxalis 2 , while Ewart (1. c.) has shown that in the case of 

 a variety of leaves that perform variation movements a reversal of the 

 photonastic response is produced by an increase in the intensity of diffuse 

 light above the optimum. It is presumably also owing to a reversal of the 

 previous heteronastic growth that, as Oltmanns found 3 , the flowers of 

 Tragopogon brevirostris close not only when the illumination decreases to 

 a minimum but also when it increases beyond a certain intensity. 



SECTION 23. The Origin of the Daily Photonastic Periodicity. 



The photonastic reactions of responsive organs are enhanced by the 

 periodicity induced by repeated previous stimulation. Hence when the 

 plant is kept in continuous constant illumination or in darkness the daily 

 movements are still performed for a certain time, but with gradually decreas- 

 ing amplitude. The periodic movements are at first pronounced both in 

 constant light and in darkness in the case of the leaves of Acacia lophantha. 

 Mimosa pudica^ Impatiens noli-me-tangere, and Sigesbeckia orientalis, and 

 they continue to be perceptible until after the lapse of four to eight days. 

 On the other hand, the daily movements of the flowers of Oxalis rosea cease 

 after being for three to four days in darkness, and the same happens in the 

 capitulums of Bellis perennis after one or two days 4 . 



After the cessation of the daily periodicity, the leaves assume positions 

 corresponding to the illumination and to other factors, while under constant 

 external conditions all movements cease except those of autonomic origin. 

 When the external conditions are favourable such leaves retain fully their 

 irritability and power of response, so that a plant whose daily periodicity 

 has been removed by continuous illumination retains its photonastic irrita- 

 bility, and responds by the usual sleep-movement when placed in darkness. 

 This was found by Pfeffer to take forty-five minutes to two hours for 

 completion in the case of the leaflets of Acacia lophantha and Trifolium 

 pratense, and of the terminal leaflets of Desmodium gyrans, whereas the 

 leaves of Impatiens noli~me-tangere sink considerably but do not attain 

 the full nyctitropic position under these circumstances. The leaves of 



1 Cf. Ewart, 1. c., pp. 448-59, 480. 2 Pfeffer> x. c>> p> 6o> 



3 Oltmanns, Bot. Ztg., 1895, p. 51. 



* Pfeffer, Period. Bewegungen d. Blattorgane, 1875, p. 34seq. 



