rapid on the first day and too slow on the second. If, however, after-effect and 

 renewed stimulus do not coincide, an after-effect, as constant as we have seen 

 exhibited in the dark by Mimosa, cannot arise. There are two possibilities by 

 way of explanation of this contradiction ; either regular and long continued 

 periodic movement in the dark is not an after-effect at all but is, at least in part, 

 a result of small oscillations in temperature which were not sufficiently carefully 

 guarded against in many of the experiments hitherto performed, or the oscilla- 

 tions observed subsequent to a single darkening in PFEFFER'S experiments, in 

 plants which had ceased to move in light, were not the result of this solitary 

 stimulus but were the after-effects of a previous daily periodicity, which they 

 had not yet quite lost, but which were only obscured in light and which had 

 lost their regularity. It would be very remarkable, however, if a single darkening 

 induced a maximum amplitude of movement, while in the case of plants which 

 had lost their power of movement, e. g. Siegesbeckia, a new stimulus took five 

 days to induce the maximum depression of the leaf. 



Seeing that objections of this kind may be advanced against PFEFFER'S 

 argument, it is necessary that experiments should be carried out on plants which 

 have never shown periodic movements. They will, we have no doubt, lead to the 

 result which PFEFFER has already indicated, namely, that periodic movements 

 arise from a summation of individual stimuli and after-effect, the single stimulus 

 inducing several oscillations which must be carried out with ever-decreasing am- 

 plitude. These oscillations following upon a single stimulus offer a subject for 

 detailed investigation. First of all they must be definitely proved (for SCHWEN- 

 DENER (1897-8) has called these facts in question, and Josx (1898) and PANTA- 

 NELLI (1900) have failed to find them in Robinia), and it must also be shown 

 how far the oscillation period depends upon the duration of the stimulus. It is 

 quite possible that we might succeed in inducing periodic movements of six hours' 

 duration or even less, but it is not outside the range of probability that the dura- 

 tion of the oscillation renders periods so short as these in the after-effects impos- 

 sible. [As a matter of fact, SEMON (1905) has succeeded in inducing a 12-hour 

 periodicity in the after-effect where, before illumination, the rhythm was 

 6-hourly or 24-hourly. All this goes to show how little indeed we do know 

 even now about periodic movements.] 



There are still many other complicated phenomena which militate against 

 a thorough explanation of periodic movement ; e. g., let us look at the case of 

 flowers and at the tulip in particular. First of all when the flower is heated 

 a single opening and closing movement only can be recognized ; no periodic 

 movements at all can be observed in the tulip. If these be possible we should 

 perhaps obtain them most speedily by alternately heating and cooling the plant 

 every hour ; but perhaps they are absent altogether. In other flowers, how- 

 ever, such movements exhibit themselves markedly. OLTMANNS (1895) draws 

 attention to many points which have as yet been but little cleared up. He finds 

 that Bellis perennis opens its inflorescence after being 48 hours in the dark, whilst 

 in the case of Tragopogon 8 to 12 hours is sufficient. He certainly regarded 

 these opening movements not as after-effects of a previous periodicity, but looked 

 upon darkening itself as the stimulus which induced opening, acting especially 

 vigorously on night-flowering plants. A discussion of this and other results 

 arrived at by OLTMANNS would lead us, however, too far ; we may merely note 

 that all these movements have not as yet been studied in a sufficiently experi- 

 mental way, and that before we can arrive at an explanation of periodicity we 

 must determine what part duration and intensity of light and of heat in the single 

 stimulus play in the process. That the origin of periodic movements from a sum- 

 mation of the individual stimuli of light and darkness together with their after- 

 effects is not quite so simple a matter as we might suppose, may be deduced from 

 the behaviour of the chief petiole of Mimosa. After a single darkening it raises 



