THE ORIGIN OF THE DAILY PHOTONASTIC PERIODICITY 109 



Sigesbeckia orientalis do not droop through an angle of more than 10 to 

 30, whereas when the normal daily movements are performed the leaves 

 droop vertically downwards at night, moving therefore through an angle 

 ofQO 01 . 



Plants exposed to the normal daily changes of illumination also afford 

 a measure of the photonastic irritability and response, for darkening during 

 the daytime produces a slight photonastic curvature in Sigesbeckia^ but 

 a pronounced one in Acacia and the other plants named above. In general 

 the photonastic reactions produced by variation movements are more rapid 

 and pronounced than those due to nutation. The pulvini of Portulaca 

 sativa only react feebly, however, whereas the nutating leaves of Impatiens 

 noli-me-tangere and of 7. parviflora are strongly photonastic 2 . In the 

 Tropics motile leaflets usually begin to assume the sleep position at about 

 5 p.m., and have completed the movement commonly by 5.30, that is half 

 an hour before the fall of night. Naturally, however, the times fluctuate 

 somewhat in different plants, and they are also affected by the clearness of 

 the sky and by the humidity of the soil and of the air 3 . 



When the periodicity has been removed by continuous illumination, 

 a photonastic reaction does not merely cause a single to and fro movement, 

 but also produces an after-effect which is naturally but slight when the 

 reaction is feeble as in Sigesbeckia. In this case the gradual return to the 

 full amplitude of movement can readily be traced as the result of the 

 co-operation of new rhythmically repeated photonastic reactions with the 

 after-effects of previous ones. Thus a plant of Sigesbeckia orientalis^ after 

 five days' continuous illumination had removed the daily periodicity, was 

 placed in darkness daily from 8 a.m. to 4 p.m. The first darkening produced 

 curvatures in the leaves of 10 to 30, the second curvatures of 15 to 45, 

 which had increased to ones of 40 to 80 on the fourth day, and of 70 to 

 100 on the fifth day. Five periodic repetitions were therefore required 

 to reproduce approximately the normal amplitude of movement in this 

 plant. 



This induction and summation cannot of course be followed when the 

 first darkening produces the full or nearly the full nyctinastic movement, 

 as in the case of the leaflets of Acacia lophantha which fold together when 

 first darkened after prolonged previous constant illumination. Even in this 

 case, however, only two periodic movements are shown as the after-effect 



1 Pfeffer, 1. c., p. 39. The plant termed Sigesbeckia flexuosa proves to be a form of Sigesbeckia 

 orientalis L. 



2 Pfeffer, 1. c., pp. 15, 39- 



3 Ewart, Annals of Botany, 1897, Vol. XI, p. 453 seq. [The midday sleep-movements of 

 Mimosa pudica and of similar plants do not appear to induce any distinct secondary periodicity, 

 although they might do so when regular and prolonged.] 



