MOVEMENTS OF IRRITATION, 483 



however, they close in the morning at 10 o'clock. The flower 

 heads of Tragopogon pratensis are open in the morning. In June, 

 however, they close in sunshine at 9 o'clock, and with a clouded 

 sky at about 11 o'clock. The flowers of Adonis vernalis, and the 

 flower heads of Bellis perennis and Leontodon hastilis close in the 

 evening and open the next morning (see Figs. 161 and 162, in 

 which are represented scapes of Leontodon in one case with a 

 closed, in the other with an open flower head). I found that the 

 flowers of Adonis, at the end of April, closed at 3 o'clock in the 

 afternoon, while flower heads of Bellis plants just near them did 

 not close till an hour and a half later. The flowers of (Enothera 

 biennis expand in the evening and close in the morning. I cut 

 scapes of Leontodon hastilis, and put them in the dark in the day- 

 time with their stalks in water. The flower heads closed in the 

 evening, but opened on the following day, although darkened all 

 the time, and in the evening once more exhibited closing move- 

 ments, though not, it is true, very vigorous ones. The next day 

 the scapes were again exposed to the daylight. In the evening of 

 that day, the flower heads closed in a perfectly normal manner. 

 These experiments teach that the individual flowers of the flower 

 heads of Leontodon hastilis and many other flowers behave in an 

 analogous manner when exposed to continuous darkness, exhibit 

 after-effect movements, which cause the flower heads to open in 

 the daytime, and close at night. It is true these after-effect move- 

 ments do not continue long. The flowers become after a time 

 motionless (darkness-rigor), but can be rendered phototonic again 

 by renewed illumination. 



From what has been said, it will now be clear that the periodic 

 movements of foliage leaves and flowers taking place under normal 

 conditions, i.e. with alternation of day and night, are not the 

 immediate result of the alternation of illumination alone. The 

 daily period is rather the resultant of paratonic effects due to the 

 alternating conditions of illumination, and after-effect movements 

 which, however, also have their ultimate origin in the alternation 

 of day and night. This also renders intelligible a fact of the 

 accuracy of which I have satisfied myself, tha/t, e.g., flower heads of 

 Leontodon hastilis placed in the dark in the forenoon, close only a 

 little earlier in the evening than others which have been illumi- 

 nated during the day. The paratonic effect of the darkening is 

 certainly to be recognised here, but the flower heads, owing to 

 after-effect's, do not close until the darkening has continued for 



