Within the unglaciated eastern United States, where the natural vegetation is mostly deciduous forest, edaphically-controlled herbaceous plant communities occur on unshaded rock outcrops in shallow soil developed from serpentine, shale, granite, limestone, dolomite, sandstone, chert, rhyolite and other rock types. The most distinctive features of the flora of these communities are their endemic taxa. Many of the endemics are not restricted to a single geological substratum, and they grow equally well, or better, in non-outcrop as in outcrop soil. Thus, it seems unlikely that they are restricted to the outcrops because of a requirement for some chemical, physical or biological element found there but nowhere else. Several of the endemics exhibit intraspecific variability in one or more of the following characteristics: morphology, physiology, breeding system, flower colour and enzyme pattern. Thus, it is unlikely that lack of genetic variation per se is the cause of endemism. A requirement that the endemics have in common for growth and survival is a high photosynthetic photon flux density. The obligate nature of the high light requirement is supported by: (1) field observations that the endemics grow on well-lighted portions of the outcrops but not in adjacent forests, (2) studies which show that growth and photosynthetic rates are maximal in high light but are severely inhibited at low light levels, and (3) field and glasshouse experiments which show that the endemics are shade intolerant and that they compete poorly with taller plants. Thus, of the three hypotheses examined to account for the proximal causes of endemism in the rock outcrop endemics, the requirement for a high level of light is the most important.