Coherent structures' dynamics in plane forced plumes is investigated experimentally by using a flow visualization and two-points measurement of velocity fluctuations and temperature fluctuations. Ensemble averages are conditioned on the occurrence of temperature fronts. The averages show that large scale coherent structures appear alternately on both sides of the centerline, which consist of vortex roll of opposite circulation each other. A flapping motion of discharged fluid is likely attributed to the asymmetric arrangement of the structures. From topological consideration, entrainment of ambient fluid proceeds by engulfment along the converging separatrix towards the saddle point connecting adjacent. same-signed vortical structures on each side.