Sink development,phyllotaxy, and dry matter distribution in a large inflorescence

This paper argues against the bicycle pump model of source-sink relationships. It suggests that it is more fruitful to examine the systems which control sink morphogenesis. This argument is illustrated by reference to the cauliflower curd (inflorescence). In any given curd the weights of successive florets (primary branches) fluctuate above and below their expected values. The fluctuations are not due to restricted vascular pipelines, nor to competitive interactions between neighbouring sinks. They are due to highly systematic fluctuations in the timing of floret initiation, and they entail similar fluctuations in primordium position. The phenomenon can be explained by assuming a control system for cytokinesis in which (i) the position of the cell plate responds to mechanical stress (ii) the degree of responsiveness varies cyclically both in time, and around the apical meristem. It seems from this and other evidence that there are strong local controls of sink development in cauliflower.