Sources and sinks vary over time depending on where nutrients are needed (what are the sinks). For example, a developing tap root is a sink until is is later used to drive growth, as in the second year of a biennial plant, when it bolts/flowers.
Work Step by Step
Review earlier chapters on sources and sinks. You will see that sugar flows to parts at some points in the year or lifecycle, and elsewhere at other times. Systemin movement should vary similarly.