Meristems, metamers and modules and the development of shoot and root systems

BARLOW, P. W., 1989. Meristems, metamers and modules in the development of shoot and root systems. Root and shoot systems are hierarchical organizations whose levels are represented, in part, by cells and meristems. Meristems produce modules which in turn construct the architectural model. The latter is species specific and its structure depends on the geometrical interrelationships between the modular elements. The place of the metamer within this hierarchical scheme is discussed. Metamers derive directly from meristem activity and are externally recognizable as reiterated sub-units of the module. Another sub-unit of module construction, the cellular complex, or merophyte, is also a product of meristem activity, but, in contrast to the metamer, it is an internal, rather than an external, anatomical feature. Being cellular, it increases the ‘span’ of the cell level rather than constituting a level in its own right. Although the physical boundaries of metamer and merophyte can overlap, or even coincide, the two units belong to different conceptual schemes of module structure: the metamer is defined from a ‘classical’ morphological viewpoint, whereas the merophyte derives from a cellular conception of plant structure. Both the merophyte and the metamer have a role in clarifying the understanding of plant development since both provide insights into the functioning of the meristems from which they are derived and the structure of the module to which they contribute. For example, modules which lack an obvious metameric construction can usefully be analysed in terms of their merophytic organization. This is particularly true of roots of lower plants. Here, the merophytes reflect the presence and activity of a specialized meristematic apical cell. On the other hand, modules of higher plants, which lack such apical cells, also lack clearly defined merophytes, but their shoots have obvious metamers which reflect the activity of the meristem as a whole. It should be possible to represent the development of modules from cells, via their intermediate sub-structures of meristems and metamers, by means of formal languages of automata theory. One of these, a graphical algorithm (Petri net), is applied in this developmental context.