Abstract. From this brief review it appears that the interactions between calcium ions and cell walls play a key role in plant physiology. Calcium ions are involved in many mechnisms: for example, stabilization of cell wall structures, acidic growth, ion exchange properties, control of the activities of wall enzymes. All these properties originate from the tight binding of calcium ions to the pectins present in the cell walls. The factor most important for controlling wall behaviour is the density of non‐diffusible charges and, due to its high affinity, calcium can significantly affect this factor. We also discuss the theoretical ion exchange models in relation to the specific role of calcium ions.

All memory functions have molecular bases, namely in signal reception and transduction, and in storage and recall of information. Thus, at all levels of organisation living organisms have some kind of memory. In plants one may distinguish two types. There are linear pathways from reception of signals and propagation of effectors to a type of memory that may be described by terms such as learning, habituation or priming. There is a storage and recall memory based on a complex network of elements with a high degree of integration and feedback. The most important elements envisaged are calcium waves, epigenetic modifications of DNA and histones, and regulation of timing via a biological clock. Experiments are described that document the occurrence of the two sorts of memory and which show how they can be distinguished. A schematic model of plant memory is derived as emergent from integration of the various modules. Possessing the two forms of memory supports the fitness of plants in response to environmental stimuli and stress.

Bacterial cells contain many large, spatially extended assemblies of ions, molecules, and macromolecules, called hyperstructures, that are implicated in functions that range from DNA replication and cell division to chemotaxis and secretion. Interactions between these hyperstructures would create a level of organization intermediate between macromolecules and the cell itself. To explore this level, a taxonomy is needed. Here, we describe classification criteria based on the form of the hyperstructure and on the processes responsible for this form. These processes include those dependent on coupled transcription-translation, protein-protein affinities, chromosome site-binding by protein, and membrane structures. Various combinations of processes determine the formation, maturation, and demise of many hyperstructures that therefore follow a trajectory within the space of classification by form/process. Hence a taxonomy by trajectory may be desirable. Finally, we suggest that working toward a taxonomy based on speculative interactions between hyperstructures promises most insight into life at this level.

It is possible to induce the formation of epidermal meristems in the hypocotyl of non‐injured and non‐hormone‐treated plantlets of flax, by combining various sorts of physical stimulations with a transient depletion of calcium. The characteristic times for the decrease of the tissue concentration of calcium during calcium depletion and for the recovery of the normal tissue concentration of calcium after resupplying the latter ion, are close to 1 day. The stimuli may correspond to wind or drought or even to the manipulation stress occurring when the plantlets are transferred from their germination to their growth vessel. Meristem production is increased by combining several physical stimulations. When calcium depletion is delayed relative to the application of the physical stimulation(s), the production of meristems is delayed accordingly. This means that the signal induced by the physical stimulation(s) may be stored within the plants, without apparent effect, until a calcium depletion finally allows the stored signal to take effect (formation of meristems). For storage periods of up to 8 days no loss of the potency of the stored signal was observed. A few other examples of storage of morphogenetic signals in plants have been described in the literature. The mechanism involved in signal storage is still not clearly understood. However, it seems that the sensing and/or storing of the signals require that the plant tissues are sufficiently rich in calcium, whereas the licensing of the plants for the translation of signals into the final response (meristem production) is done by a transient calcium depletion.