THE ORIGIN OF THE DAILY PHOTONASTIC PERIODICITY in 



the swings of a pendulum. The rhythm of a simple pendulum is constant 

 so long as its length and the force of gravity are unaltered. On the other 

 hand, the photonastic rhythm of a living organ can be made to follow 

 periods of more or of less than twenty-four hours by corresponding 

 alterations of the periods of illumination and darkness *. 



It is worthy of note that the nyctinastic periodicity vanishes com- 

 paratively rapidly and hence never becomes hereditary, although it may 

 have been regularly repeated through countless generations. Only in 

 very few cases, in fact, is a hereditary transmission of a long induced 

 periodicity possible. This actually applies to the resting and flowering 

 periods of certain plants, for when transferred to other climates, a new 

 hereditary rhythm may be gradually induced which is appropriate to the 

 altered seasons. 



The spontaneous movements of the leaflets of Oxalis, Trifolium, and 

 of the terminal leaflet of Desmodium gyrans retain the same rhythm of 

 forty-five minutes to four hours under continuous illumination, whereas the 

 periodic nyctinastic movements gradually cease. Hence the latter cannot 

 be derived by the regulation of the spontaneous movements 2 , although in 

 other cases a particular rhythm may result from the regulation of an 

 inherent periodicity, as is in part the case with the yearly periodicity. 



The power of photonastic response is not necessarily coupled with 

 a pronounced thermonastic irritability, and most photonastic organs are 

 irresponsive to mechanical stimuli such as produce pronounced movements 

 in the leaflets of Mimosa and Oxalis z . Pronounced spontaneous move- 

 ments are shown by certain leaves, but are absent from most organs 

 capable of sleep -movements such as the leaves of Acacia lophantha, 

 Impatiens, and Sigesbeckia, while the lateral leaflets of Desmodium gyrans 

 which show rapid spontaneous movements perform no sleep-movements 4 . 



Historical The sleep-movements of certain plants were first noted by Pliny, and 

 by Albertus Magnus, but Linnaeus was the first to call attention to the common 

 occurrence of nyctinastic movements among leaves and flowers 5 . The subsequent 

 researches, which were mainly concerned with the mechanics and causes of the 

 phenomenon, left it uncertain whether the daily rhythm was due to the periodic 



1 Cf. Pfeffer, 1. c., pp. 39, 43, 53. The time of reaction naturally sets a limit to the possible 

 shortening of the rhythmic period. 



2 Pfeffer, 1. c., pp. 35, 52. 



3 Conversely mechanically irritable organs such as the stamens of Cynareae, various stigmas and 

 tendrils, perform no sleep-movements, and the same applies to the leaves of Dionata (Munk, Die 

 elektrischen u. Bewegungserscheinungen von Dionaea, 1876, p. 101), and of Drosera rotundifolia 

 (Kabsch, Bot. Ztg., 1860, p. 247). 



* Darwin, The Power of Movement in Plants. 



5 For details on the historical development of this subject see Pfeffer, Periodische Bewegungen, 



l8 75 PP- 3> l6 3- 



