no MOVEMENTS OF CURVATURE 



of a single reaction, whereas when the daily periodicity is fully induced four 

 or five periodic movements may be shown under constant illumination. 



It is evident, therefore, that the nyctinastic periodicity is induced by the 

 rhythmically-repeated photonastic reactions and their after-effects. The 

 daily periodicity of growth is produced in a similar way, and a photonastic 

 periodicity must always result from the rhythmic and regular repetition 

 of changes of illumination whenever these affect either growth or the energy 

 of expansion of motile tissues \ The after-effects of photonastic stimulation 

 enable a phototonic plant to perform movements of considerable amplitude, 

 although the primary movement directly due to the photonastic stimula- 

 tion may be comparatively feeble, and in addition a plant with a pronounced 

 periodicity of this kind will tend to be more regular in its daily movements 

 than if nearly the full movement was produced in response to a single 

 change of illumination. 



An analogy is afforded by a pendulum whose amplitude of oscillation 

 is gradually increased up to a maximum by a series of rhythmically-repeated 

 impulses, and which then continues to oscillate with gradually decreasing 

 amplitude but without any appreciable change of period 2 . In the living 

 plant, although the cumulative after-effects of the previous rhythmic stimula- 

 tion may be phenomena of complex origin, we can nevertheless deal with 

 them as with other empirically established facts. Not all movements or 

 stimulatory reactions are able to exercise appreciable after-effects, and since 

 when they result from a particular reaction they may vary in character 

 according to the nature of the plant and its power of reaction, it is to be 

 expected that specific peculiarities should be shown in regard to the after- 

 effects of photonastic stimulation. In fact they may persist for a long time 

 in some plants but only for a day in others, even when they had been per- 

 forming pronounced sleep-movements every night during the whole of their 

 adult existence. In addition, no periodic after-affects appear to be produced 

 in photonastic flowers by the alternation of night and day, and little or no 

 after-effect appears to be exercised by the pronounced thermonastic opening 

 and closing movements of the flowers of Crocus and Tulipa 3 . 



The periodic after-affects when present follow approximately the same 

 rhythm as the nyctinastic movements which give rise to them, and hence 

 the one aids the other. The times of oscillation of a simple pendulum 

 swinging in still air vary, somewhat according to their amplitude, and the 

 successive after-effects of periodic stimulation are still less isochronous than 



1 On the feeble periodicity induced by rhythmically-repeated geotropic or heliotropic stimuli, cf. 

 Darwin and Pertz, Annals of Botany, 1903, Vol. xvii, p. 93. 



2 Pfeffer, 1. c. It is difficult to understand how Schwendener (1897, Gesammelte bot. Mittheil., 

 Bd. II, p. 241) can be in any doubt as to the propriety of using this analogy with a pendulum as an 

 illustration of the nature of periodicity and of periodic phenomena. 



3 Pfeffer, 1. c., p. 133 ; Jost, Jahrb. f. wiss. Bot., 1898, Bd. xxxi, p. 349. 



