I feel near to sure this is Begonia hirsuta, the only problem is that according to the 'Flora do Brasil' the plant doesn't occur in the Rio de Janeiro State, but in the immidiate neighbouring state: Espirito Santo. As I found the plant in a touristic preserved part of the Atlantic Forest (Rio de Janeiro) with daily visitors mainly Brazilians from different parts of the country, I find it very possible the dust like seeds have travelled...
The 'Flora do Brasil' also sees now Begonia hirsuta as a synonym of Begonia humilis var. humilis, but the Plantlist not..
http://floradobrasil.jbrj.gov....
http://www.theplantlist.org/tp...
I recognize and based my identification of my plant on this;
https://commons.wikimedia.org/...
they got their material from the National Botanical Garden of Belgium..
I know it also looks similar to Begonia hirtella but I've seen both in person on different occasions and there are slight differences like the flowers of B.hirtella are bigger, the ones on ‘my’ plant are very tiny and another distinguishing feature is that the flower stems on my supposed B. hirsuta are always red and the base of the leaves with a red dot and more undulated between the veins..
The first photos are in the location where I found them:
The next lot are from plants I've grown from the seed I collected at the time, (don’t ask me why, the plant is not showy at all, but it had seedpods so couldn’t resist!
)
