Losada H, Vieyra J, Rangel J, Zamudio A y Martínez G 1997: The sub-urban agro-ecosystem of maize production in the south-east hills of Mexico City as an important cultural and sustainable crop resource. Livestock Research for Rural Development. Volume 9, Article #44. Retrieved , from http://www.lrrd.org/lrrd9/5/tito951.htm