120 MOVEMENTS OF CURVATURE 



Pfeffer 1 , but sufficient attention has not always been paid to this by 

 Oltmanns 2 in his interpretation of the opening and closing movements of 

 flowers. 



Since the power of reaction is always present, numerous and often pro- 

 nounced oscillations may occur as the result of variations of illumination 

 during the day, especially in the case of organs exhibiting strong photonastic 

 irritability. Darkening at midday produces, however, more effect than in 

 the morning, since in the first case the photonastic response is aided by the 

 incipient periodic after-effect. Hence the appearance of thunder-clouds at 

 midday may cause the leaves and flowers of many plants to perform sleep- 

 movements, whereas the same darkening during the early morning may only 

 induce a feeble closing movement 3 . If, however, the periodic after-effect is 

 strong, but the direct photonastic reaction feeble, darkening in the morning 

 may cause an only temporary retardation or reversal of the opening move- 

 ment, which is ultimately resumed and completed 4 . 



Illumination during the evening closure acts in the same manner, and 

 in strongly photonastic plants such as Mimosa and Acacia the leaves may 

 be brought back into the expanded position by illumination applied at the 

 close of a cloudy day 6 . If a plant is illuminated during the night and 

 darkened during the day, a rhythm corresponding to the altered conditions 

 will be more or less rapidly induced after a few irregularities, and the new 

 rhythm may be capable of persisting for more or for less than a day. 



Since some time is required for the accommodation to the new conditions, 

 a previously darkened plant must be exposed to light for some time before renewed 

 darkening is able to produce a perceptible response. The leaves of Acacia lophantha, 

 and of Impatiens noli-me-tangere, are able to show a feeble photonastic response to 

 darkening after five to ten minutes' illumination, and after thirty minutes to an 

 hour's exposure they are capable of exhibiting a maximal photonastic response, which 

 undergoes no further increase, even after prolonged constant illumination 6 . [The 

 photonastic response to intense illumination is much more rapidly produced, and in 

 this way its utility as a protection against temporary intense exposure is considerably 

 enhanced. Thus, in highly turgid plants of Mimosa pudica, the leaflets begin to fold 

 together a second or two after strong sunlight falls upon them, and in thirty seconds 

 to a minute become completely folded. The re-expansion in weak diffuse daylight 

 takes from one to three minutes after short exposure, but a longer time is required 

 when the exposure has been more prolonged. After midday the responses are 

 usually less rapid, but this appears to be due merely to the lessened turgidity. 

 Owing to the existence of a latent period, and an after-effect, the stimuli due to short 

 periods of exposure may be summated so as to produce a response, and for the same 



1 Period. Bewegungen, 1875. a Bot. Ztg., 1895, p. 44. 



3 Pfeffer, Physiol. Unters., 1873, p. aoi. 4 L. c., 1875, p. 71. 



8 This opposed action was utilized by Pfeffer to produce a more rapid elimination of the daily 

 periodicity under continuous illumination (1. c., 1875, pp. 35, 71). 

 6 Pfeffer, 1. c., 1875, p. 57. 



