PLANTS AS COMPETING POPULATIONS OF REDUNDANT ORGANS

At any given time, a vascular or land plant may be a colony of functional sectors, each consisting of a shoot and its associated roots. In most plants, however, the activity of the cambium can change the relative vascular contacts of neighbouring shoots. Vascular tissues can even differentiate along new orientations, forming contacts that change the sectorial structure of the plant. Such reoriented differentiation is induced by the same auxin from developing leaves as are other types of vascular differentiation. The occurrence of vascular reorientation is determined by two criteria: the presence of an auxin flow that exceeds the transport capacity of the tissues that follow the previous, established orientation and the availability of nearby channels that are not fully occupied, not 'protected' by their own now of auxin. These controls of vascular orientation suggest that neighbouring shoots (and neighbouring roots) compete with one another, by means of signals indicating their state and their environment, for vascular contacts with the rest of the plant. Such internal competition between genetically equivalent shoots is an adaptation to heterogeneous environments: it is the shoots in the best conditions available to the plant that receive the support of a greater part of the root system. The potential for changes of vascular contacts points to open problems and to neglected aspects of the role of the cambium in plant organization.