During centuries, a "vital strength" was assumed to have conferred on inanimate matter the property of becoming alive and of continuing to promote the spontaneous generation of living organisms from non-living material; however, not everyone agreed with this. The controversy continued to increase until the nineteenth century when all the experiments purporting to prove spontaneous generation were shown to be erroneous [see, for instance, Spallanzani (1787) and Pennetier (1907)]. Chemists succeeded in the abiotic synthesis of organic compounds (thus demonstrating that the substances produced by living processes and by artificial synthesis were not different in essence) and the theories of vitalism and spontaneous generation were abandoned.
Even before the old beliefs were rejected, a new approach, often termed "reduc-tionism", had begun to be developed. The aim was no longer to obtain a global interpretation of life, but to describe biological objects at the molecular, cytological and histological levels and to study how the structure of each object endowed it with its particular function. This resulted in the acquisition of fundamental biological knowledge and in the development of innovative agricultural, medical and industrial applications. However, it was also clear that the reductionist approach was not sufficient to give a fully satisfactory answer to the question "What is life?"
This answer entailed understanding not only the function but also the integrated functioning of biological objects. The availability of isotopic tracers, the increasing performance of computers and conceptual advances (e.g. the "Catastrophe theory" by Rene Thom, the "Dissipative structures" of Ilya Prigogine and the general theories of non-equilibrium thermodynamics and of dynamical systems) progressively provided researchers with scientific tools more efficient than those available to the physiologists of the previous centuries.
Our group has used these tools to address the problem of the nature of life [97,108] in four complementary ways: (a) methodological improvements (and their practical applications), (b) kinetics of enzyme-catalysed reactions under unusual conditions (including the case of "functioning-dependent structures" and their role in cell regulation), (c) solute fluxes and transmembrane transports, (d) signalling and response to stimuli. We have used plants as experimental models in most cases. I have collaborated with many co-workers. Their contributions have always been valuable and sometimes essential. Their names appear in the list of our major publications, which rank in chronological order in the Appendix.
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