The hypocotyls of Arabidopsis thaliana seedlings exhibit circumnutations with different frequency ranges. Using a picture-analysis system, two types of oscillations were distinguished, short period (SPN) and long period nutations (LPN). The period of the SPNs is between 20 and 60 min, decreases with increasing temperature (between 20° and 30°C; Q 10 of 2.0) and increases with age. The SPNs changed frequently between circular and pendular movements. The circumnutation usually occurred clockwise, but the direction could change. For SPNs to occur, a minimum growth rate of 0.05 mm/h was required. The period of the LPNs ranged from 1 to 8 h. Peaks occurred around 95 and 200 min. The movements of the LPNs are mostly counterclockwise. The oscillation could change from circular to elliptic or pendular. In contrast to SPNs, LPNs were found also at low growth rates. Under red light and in the strain Landsberg erecta the circumnutations were especially slow. Short and long period oscillations can occur simultaneously or separately in different plants. In cases of simultaneous occurrence the quotient between the period lengths of the LPN and SPN was not constant. The number of occurrences of these frequencies depended on the strain and on external factors such as temperature and light. In continuous bright white light phases of circumnutations alternated with quiescent periods. This behaviour was circadian and correlated with growth bursts.

A circadian rhythm o f leaf movements o f Arabidopsis thaliana and its recording in continuous weak light with a video-computer system is described.

Summary. Phase shifting and abolishing the circadian oscillation which controls eclosion in Drosophila pseudoobscura does not shorten the life of the flies and does not impair eclosion out of the puparium. However, the activity rhythm of the flies is controlled by a circadian oscillator which differs in period length, sensitivity to light and other features from the one controlling eclosion. Therefore, perturbing the oscillation controlling eclosion does not necessarily imply a corresponding perturbation of the oscillation controlling activity. Time Flies Like an ArrowBut Fruit Flies Like a Banana *** damp out circadian rhythms-damage the plants (Went, 1962; Hillmann, 1956). Administration of phenobarbital in the diet of rats causes "dyschronism" and life shortening (Ehret et al., 1978). It was inferred from these experiments that circadian rhythms are important for the phased maintenance of physiological processes and that disturbances reduce life expectancy. We have attempted to test this hypothesis further in a more rigorous way. We used the findings of Winfree (1970) to abolish the circadian rhythm of eclosion by a perturbation with a light pulse of a specific intensity, duration, and wavelength at a specific time during the daily cycle. If circadian rhythms are indeed important in upholding the physiological integrity of the organism, the abolishing of the rhythm might lead to a decreased longevity.The functional significance of circadian rhythms was tested by Aschoff et al. (1971) in experiments in which adult flies of Phormia were treated with light/dark cycles in such a way that they were frequently phase shifted simulating rapid east-and westward travel through time zones. The longevity of these groups was shorter than controls. The same was found by Halberg et al. (1975) for mice. Likewise, Pittendrigh and Minis (1972) showed that driving the circadian oscillation of Drosophila melanogaster with periods away from the normal 24 h decreases longevity. In tomatoes and other plants constant conditions of temperature and continuous light-which are known to * Dedicated to Professor Colin S. Pittendrigh on the occasion of his 60th birthday ** With the participation

The circadian petal movement rhythm of Kalanchoë flowers has been studied. The amplitude of the rhythm can be drastically reduced by an appropriate stimulus of a light pulse. It has also been shown that it is possible to stop the rhythm permanently by administering a single light pulse to the flowers. This is interpreted to indicate that the light pulse has sent the circadian rhythm into a stable state of singularity. The conditions which attenuate the rhythm have been investigated both theoretically (on the basis of a previously published model for circadian rhythms) and experimentally. 120 min red light of 230 μW · cm−2, starting briefly before the second petal closure about 30 h after transfer to constant safe light conditions is optimal in inducing rhythm‐damping. Damping requires the same duration when the light is given at the corresponding phase during the third or fourth cycle of the rhythm. However, in the first cycle 240 min red light of 230 μW · cm−2 is required to get optimal damping of the rhythm. Conditions to achieve damping for other irradiances are investigated. Individual recordings are presented which show the behaviour of the rhythm when perturbed by light stimuli close to its singularity.

Unter the permanent influence of lithium ions the circadian rhythm of movement of Kalanchoe petals is effectively lengthened but a pulse administered up to 12 hours has no influence. Lithium ions could also be demonstrated to similarly slow down the circadian activity rhythm of a small mammal (Meriones crassus). It is hypothesized that the therapeutic effect of lithium salts in endogenous depressions in human beings may result also from its acting on the human circadian system