INSTANCES OF PHOTONASTIC AND DIURNAL MOVEMENTS 105 



of Lupinus become erect at night-time, and those of other species droop downwards. 

 Darwin also gives instances of plants in which the cotyledons perform different sleep- 

 movements to the foliage-leaves, and mentions that the young cotyledons of Trifolium 

 strictum, in addition to the pulvinar movements, show a torsion of the petioles '. In 

 addition, the position of the leaf of Bauhinia changes at night-time, while the two 

 halves of the leaf fold together along the midrib. (Cf. Fig. 31, p. 107.) In many 

 cases, though not always, the sleep position of the leaves resembles that which they 

 occupied during their early development 2 . 



Since these daily movements are the result of a photonastic reaction 

 coupled with the after-effect of periodic stimulation, every increase or 

 decrease of illumination produces a more or less pronounced movement 

 corresponding to that occurring at morning and evening respectively. In 

 both cases the movement surpasses the permanent position of equilibrium, 

 and that often to a considerable extent. Hence it arises that periodic 

 flowers are usually most widely open during the early morning hours. 



Changes of illumination always affect the position of the leaves more 

 or less. The latter is more especially the case with the leaves and leaflets 

 of Phaseolus and Acacia lophantha, for when these are placed in darkness, 

 after performing a photonastic movement they return approximately to their 

 original position, and maintain this position, in part at least, for an indefinite 

 length of time in continued darkness. On the other hand, amongst others, 

 the leaves of Impatiens noli-me-tangere and of CJienopodium album when 

 placed in darkness during the daytime assume a position intermediate 

 between the day and night positions, and one which approaches the former 

 or latter more closely according to the species of plant. Flowers which 

 perform nyctinastic movements show similar peculiarities 3 . 



Photonastic changes of position are, however, also shown by organs 

 which do not exhibit any pronounced daily sleep-movements, either because 

 they do not react rapidly enough, or not in such fashion as to produce 

 a definite movement. Reactions of this kind take part in and often are 

 mainly responsible for the assumption of the permanent position of dorsi- 

 ventral organs. Heteronastic, tropic, and other reactions may, however, 

 also co-operate with the photonastic, thermonastic, and hydronastic responses. 

 The leaves of Taraxaciim officinale, Plantago media, and Primula elatior 

 assume a more or less erect position in deeply shaded places, but under 

 strong diffuse illumination spread more or less horizontally, so that the 

 rosette of leaves is often closely applied to the ground. Light therefore 

 favours epinastic and darkness hyponastic growth in these leaves, and when 



1 Darwin, 1. c., pp. 309-13. 

 a Cf. Pfeffer, 1. c.; Dietz, Flora, 1887, p. 577. 



3 Cf. Pfeffer, 1. c., pp. 19, 38, 49. Autonomic changes of position are possible under constant 

 external conditions. Cf. Pfeffer, 1. c, p. 49, &c. ; Jost, Jahrb. f. wiss. Bot., 1898, Bd. xxxi, p. 382. 



