Abstract
We have studied the closest bright planetary nebulae with the Hubble Space Telescope's WFPC2 in order to characterize the dense knots already known to exist in NGC 7293. We find knots in all of the objects, arguing that knots are common, simply not always observed because of distance. The knots appear to form early in the life cycle of the nebula, probably being formed by an instability mechanism operating at the nebula's ionization front. As the front passes through the knots they are exposed to the photoionizing radiation field of the central star, causing them to be modified in their appearance. This would then explain as evolution the difference of appearance like the lacy filaments seen only in extinction in IC 4406 on the one extreme and the highly symmetric ``cometary" knots seen in NGC 7293. The intermediate form knots seen in NGC 2392, NGC 6720, and NGC 6853 would then represent intermediate phases of this evolution.
