The microstructure of thermal convection at large Rayleigh number was studied experimentally. Sheetlike plumes generated at the heated bottom plate are composed of several thermals generated successively. The projection of the sheetlike plumes on the bottom plate is called convection lines. On the other hand, in the area surrounded by sheetlike plumes where descending compensational flow prevails, the conduction layer always remains in the vicinity of the bottom, within which excess temperature is still high. The heat supplied from the bottom is transported horizontally by weak flow in the conduction layer toward the convection lines where rising sheetlike plumes carry the converged heat into the main body of water. Fluctuation of the convection lines was investigated. It is found that phenomenologically, the merging of thermals causes much stronger sheetlike plumes and accelerates the formation of largescale convection. ¿American Geophysical Union 1987