Fundamental mechanisms of sheetlike plumes generated on a heated bottom plate have been studied experimentally by flow visualization and confirmed by temperature measurements. The range of the flux Rayleigh number was between 108 and 1012. The Prandtl number was between 6.0 and 8.7. The values of aspect raio were 3 to 50. A plan of sheetlike plumes adjacent to the bottom plate takes a polygonal pattern, and these sheetlike plumes are named bursting lines in this paper. The average length of the side of these bursting lines is expressed as a sole function of flux Rayleigh number Raf when the length is nondimensionalized by fluid depth. It is confirmed that sheetlike plumes consist of a set of thermals that are repeatedly discharged along bursting lines and that essentially show a stochastic nature from a microscopic point of view. It is also shown that the nondimensional frequency of the production of thermals is proportional to Raf, and the dimensionless duration is inversely proportional to the square root of Raf. The accordance of intervals of thermals in the initial stage with those in the quasi-steady convective motion infers that the generation mechanism of thermals remains unchanged under the existence of global circulation.