In pigeonpeas (Cajanus cajan), most flowers are shed without setting pods. Pod-set is reduced by shading, defoliation and the presence of already developing pods, probably because of the reduced availability of assimilates or other nutrients. In pigeonpeas, unlike most leguminous crops, the average weight per pod of earlier and later formed pods is the same; this indicates that pod-filling is not limited by nutrient supply. Pod-set seems to be controlled in such a way that fewer pods develop than the plants are capable of filling. These processes can be represented by a simple working model, in which the assimilate supply corresponds to water in a reservoir, the axis of a branch or a raceme to a horizontal tube connected to the reservoir, and pods to containers of limited volume at a lower level; the connecting tubes between the axis and the 'pods' have an ascending limb, shorter than the descending limb to the pods, creating a siphon. 'Pods' can 'set' only when the level of water in the reservoir is higher than the threshold of the siphon; during the filling of earlier-set 'pods', the setting of other 'pods' is inhibited by the reduction of pressure within the axis. This model may provide a crude representation of mass flow within the phloem from sources to sinks; it also illustrates some of the hydrodynamical factors involved in competition among sinks
