Dear Cali and Alisdair,
Thank you for your answers. I must now plan another trip to Great Britain to visit this Collinson's Garden.
I have something else to add about Andrea Wulf's book. I was feeling involved when I learnt that Bartram discovered Franklinia alatamaha and that this shrub is extinct in its habitat (Georgia, If I remember). All the specimens around the world are coming from the seeds Bartram's son collected.
When we visited a garden in Oregon in 2008, I stood completely amazed in front of the flowers of this shrub. By chance, there were some seed pods and, so, I picked up some. It is the reason why, you can find on the MGS seed list, the famous Franklinia named after Benjamin Franklin who was Bartram and Collinson's friend. Unfortunately, I only send one order of these seeds. Of course, it is not very mediterranean and it must be the reason.
When I think that these seeds are the lineage of Bartram's plants, I am touched with emotion.
Cali, I put this other book on my whish list, but as I don't read very quickly in english (imagine, I have the big Harraps dictionary close to me, that I have to consult from time to time), I'll wait for a while.......