In the luminosity functions (LF's) of the HII regions of a set of late-type spiral galaxies we have identified the presence of a phase-change via a jump in the function accompanied by a change in slope.The nature of the phase-change is diagnosed using measurements of the internal radial surface brightness of individual regions.Plotting the internal brightness gradient against luminosity we find a jump in gradient at the same luminosity as the glitch in the LF.A reasonable hypothetical explanation is that we are seeing the effects of the change from ionization bounding to density bounding:the regions with luminosities higher than the transition value are density bounded.We show that the condition for this to occur at a well-defined luminosity is that the slope of the LF below the transition be less than the slope above it,and demonstrate that this condition is fulfilled in all the observed cases.The physical basis for this explanation is that the star -formation process causes the ionizing photon flux in a young massive stellar association to vary more rapidly than linearly with the mass of its placental cloud. If our hypothesis is correct, the LF of the regions above the transition should in fact be a measure of the mass function of the placental clouds in this range.By comparing the LF slopes below and above the transition we can compare the mass function of the stellar component with the mass function of the gas clouds from which this forms, and thereby set important constraints on the physics of the massive star formation process.We give the intial results of computations to this end for the set of galaxies observed and analyzed, and suggest further tests to check the underlying theory,which require comparisons of fluxes in prescribed emission lines from regions below and above the transition.