They do vary according to age Myriam, the growing conditions might make a difference too.
There's more photos showing the rounded leaves on Marcgravia polyantha which have a paler edge. It's amazing how they change from growing on a tree trunk to a shrub, they are 'hemiepiphytes'.
http://www.ufrgs.br/fitoecolog...
Marcgravia polyantha is listed on the following link which I started to go through but didn't get that far ..
http://www.scielo.br/scielo.ph...
Just saw your post, Marcgravia umbellata does look a better fit! It's not far away so could be the one.
