Abstract Individual cells constitute the minimal organization level to generate ultradian rhythms. A cell biology approach is thus necessary to better understand the intrinsic nature of these natural oscillators and their evolutionary significance. In this respect, pollen tubes provide a useful working model because, unlike other cells, their growth can be conveniently followed in vitro and it is known to involve both structural, biochemical as well as biophysical oscillators. As in any other complex system, these oscillations involve almost all cellular components but, in this case, no causal role has yet been identified. Most studies consider growth as the reference for statistical correlation analysis with other oscillating parts, interpreted as an effect if correlated before growth and as a consequence when correlated after growth. Today, it is known that this group of oscillating variables include at least ion fluxes and internal free concentrations (calcium, chloride, protons and potassium), the cytoskeleton, membrane flow and wall synthesis. Despite the progress made in this domain, however, a central core-controlling mechanism is still missing, and even less is know about how all components interact to produce the macroscopic outcome, i.e. structural organized apical growth. In other words, we can see the arms of the clock and many underlying moving parts but still miss which work as pendulum, escapement and anchor. Here, we review the recent advances in this field and critically address some of the pitfalls and inconsistencies in the data presently available. Some conceptual outlines and future directions of research are also discussed. 2.1 Finding Stability in Instability For a long time, the view of a biological system as an equilibrium state- dependent structure has led researchers to use discrete time intervals and to statistically treat the average of the observations as a close reflection of reality. S. Mancuso and S. Shabala (Eds.) Rhythms in Plants: Phenomenology, Mechanisms, and Adaptive Significance © Springer-Verlag Berlin Heidelberg 2007 2 The Pollen Tube Oscillator: Integrating Biophysics and Biochemistry into Cellular Growth and Morphogenesis NUNO MORENO 1 , RENATO COLAÇO 1 AND JOSÉ A. FEIJÓ* 1,2 1 Centro de Biologia do Desenvolvimento, Instituto Gulbenkian de Ciência, 2780-156 Oeiras, Portugal 2 Universidade de Lisboa, Faculdade de Ciências, Dept. Biologia Vegetal, Campo Grande C2, 1749-016 Lisboa, Portugal *Corresponding author, e-mail: jfeijo@fc.ul.pt Ch02.qxd 18/12/06 11:57 AM Page 39