For instance, rhythms that occur at markedly longer intervals than a day, such as tidal (lunar) rhythms, the "periodical cicadas" (Simon and Cooley [46]) and some flowering rhythms are ignored. In the case of ultradian rhythms, one can come up with an explanation that posits a selective advantage of the rhythm I per se i , with the precise reason behind the advantage depending on the rhythm in question (e.g., Nanjundiah [38]). Keywords: Biological morphology; cell-cell interactions; cellular growth; development; emergent behaviour; intercellular signalling; oscillations EN Biological morphology cell-cell interactions cellular growth development emergent behaviour intercellular signalling oscillations 1 7 7 09/12/23 20231201 NES 231201 Dedicated to the memory of Wolfgang Engelmann: warm friend, excellent scientist, eminent teacher. Ultradian/circahoralian rhythms are described in four chapters, focusing on the presentation of cellular rhythms, the nature of the rhythms (e.g., a discussion on their fractal structure) and search for the mechanisms ("the pacemaker"). [Extracted from the article]