A large cycad, Dion edule, growing near Jalcomulco in the state of Veracruz, Mexico. I conducted a series of long-term ecological studies on this species in the mid-1980's with Andrew Vovides from the Instituto de Ecologia in Jalapa. We monitored the growth and reproduction of marked individuals for several years and discovered some fascinating facts about this primitive plant.
We found out, for example, that some of the individuals on our study site were more than 2,500 years old, and that Dion edule is the oldest living plant in Mexico. Given their great age and small stature, we also estimated that these are some of the slowest growing plants in the world. A more detailed description of the Dion edule study can be found here. [NOTE: Soon after we published our findings, Dion edule was listed in the Guinness Book of World Records as the slowest growing plant in the world. I couldn't find a link to the Guinness book, but the species is listed in several other places (e.g. here) with this distinction.