References

A general reference for the Root effect is:

Chapter 4: The Physiology of the Root Effect by B. Pelster and D. Randall, in the book:
Fish Respiration, S. Perry and B. Tufts, eds.; Academic Press 1998
You can probably get it from interlibrary loan from your local library.

The recent paper on the evolution of the Root effect is:

Evolution of Oxygen Secretion in Fishes and the Emergence of a Complex Physiological System by Berenbrink, Koldkjaer, Kepp, and Cossins published in the journal:
Science Vol 307, 18 March 2005, pg 1752-1757

The journal Science is available in many public and all University libraries. Its web site is:
http://www.sciencemag.org

At the web site you can purchase one article for $10 (of course you can also subscribe to the journal for one year for a much larger fee). If the article is more than a year old you can download it for free, otherwise you can't even see the abstract of an article for free. This is somewhat curious since Medline provides abstracts for all journals <link>.