Figure. 12 Autoradiographs of a leaf of summer squash (Cucurbita pepo showing the transition of the leaf from sink to source status. In each case, the leaf imported 140 from the source leaf on plant for 2 hours. Label visible as black accumulation (A) The entire leaf is a sink, importaning sugar from the source leaf. (B-D) The base is still sink. As the tip of the leaf loses the ability to unload and stops importing sugar (as shown by the loss of black accumulation), it gains the ability to load and to export sugar. (From Turgeon and Webb, 1973)