The total intensity of light scattered by interplanetary dust has usually been directly related to the dust mass lost because of the Poynting-Robertson effect. The scattered light intensity thus would appear to define a dust supply rate that must be maintained if the dust intensity is to remain constant. This work suggests that comets apparently cannot supply this quantity of dust to the cloud, because radiative pressures prevent most of the comet debris and its secondary collision fragments from entering closed orbits about the sun. More observations on the actual emission rates of very-short-period comets may be needed before this can be conclusively established. It is shown that asteroidal collisions can provide sufficient debris, but the injection rate is extremely variable because most of the debris must be produced in very rare collisions between the largest asteroids. It is concluded that no current theory is capable of accounting for a steady cloud. A possible solution might be found in processes that would counter the Poynting-Robertson drag and permit grains to be long lived. The present comet dust supply rate might then be adequate to account for a stationary cloud. An alternative is a highly variable cloud, with dust produced in asteroidal collisions. The present epoch would then have to be one of unusually high dust density,