1. How many plant species are there;Bramwell D.;Pl. Talk,2002
2. CBD (Convention on Biological Diversity) 2003 Global strategy for plant conservation. Montreal Quebec: Secretariat of the Convention on Biological Diversity.
3. Darwin C. 1881 Letter: to my executors and other children (held in Darwin Archive Cambridge University Library DAR 161:80). Transcription personal communication. Cambridge: Charles Darwin Correspondence Project. See http://www.lib.cam.ac.uk/Departments/Darwin/.
4. Frodin D. & Govaerts R. 1996 World checklist and bibliography of Magnoliaceae. Kew UK: Royal Botanic Gardens.
5. How many species of seed plants are there?