1. Levin, D. A. The nature of plant species. Science 204, 381–384 (1979)
2. Raven, P. H. in Modern Aspects of Species (eds Iwatsuki, K., Raven, P. H. & Bock, W. J.) 11–29 (Univ. of Tokyo Press, Tokyo, 1986)
3. Hickman, J. C. The Jepson Manual of Higher Plants in California (Univ. of California Press, Berkeley, 1993)
4. Bachmann, K. Species as units of diversity: an outdated concept. Theory Biosci. 117, 213–230 (1998)
5. Mishler, B. D. in Species: New Interdisciplinary Essays (ed. Wilson, R. A.) 307–316 (MIT Press, Cambridge, Massachusetts, 1999)