abstract: Leaflet movements of two shadowplants, Oxalis acetosella and O. regnellii, has been recorded for one year in natural light-dark changes at Messaure (66°42'N,20°25'E). From March until the middle of September, starts and ends of the leaflet movements were correlated to sunrise and sunset (Daily 5-Lux thresholds) respectively. From the middle of September (October) starts and ends of the leaflet movements were running in opposite direction to what starts and ends of day did. This resulted in an increase of the duration of day-positions from September until December. Oxalis acetosella thereby reached day-positions as long as in summer. Duration of day-position, represented as a function of duration of day, showed an unsymmetrical S-curve, withdrawing far away, from the proporionality-line during the winter time. As possible causes for these phenomena, increasing sensibility of the plants to the Zeitgeber and regulation by two oscillators has been discussed.
authors.affiliation: Messaure Biological Station, Messaure, Schweden.
authors.forename: Agnes
authors.initials: A
authors.lastname: Müller-Haeckel