An equation of the form y=kx to the z power describing more precisely the species-area relation for plant species in smaller areas is proposed as a result of intensive examination of species presence in six stands of three undisturbed plant communities in the midwest. Each individual equation for each stand is based on a relatively small sample of 900 m2, but within such an area, the fit of observed data to the equations is very close. In these equations, in which y represents species numbers and x the area, k varies primarily according to species size, while the exponent z more truly reflects the species-richness of the community. Both of these constants may prove to be important measures of different community features. The equations fit rooted data more closely than covered, and it is suggested that the former type of information, being easier to obtain and probably more accurate, is preferable. Comparison of published species numbers for larger areas in floras shows clearly that the above equation, so very accurate in the smaller areas actually sampled, does not hold for larger areas as the actual number of species drops far below what the equation predicts. An equation describing this relation for a community over its entire range of area cannot be accurately constructed without considerable additional information on species numbers in larger areas. Such an equation will probably be sigmoid and more complex than the ones presented here. A method is described whereby one may objectively select an appropriate plot size for a given community based on its equation and not affected by the size of the sample from which the equation is derived. Such equations provide a useful means of comparing several floristic features as well as helping to assess historical aspects of plant communities.