To Jean-Claude Parisot, Adama Barry and Salif Napon for the access to analytical collections from Gangaol, Perkoa and Pagala. To Seth Godonou and Issouf Ouédraogo for their useful remarks of reader.

I am particularly indebted to Hélène Paquet for the English translation.


This synthesis was conducted as part of the IRD (ex ORSTOM) in Ouagadougou (Burkina Faso).