[/i][/color]

I'll try again . . .
I am still trying to identify this plant.
I have added three photos to the link (I now have much more respect
for plant photographers - it is not easy !)
One shows the "opposite" leaf arrangement and how new shoots grow out
of the "armpits" -
rather like tomatos.
Another shows the tiny purple/blue flowers about to come out.
At this stage the bottom part of the main stem has (often?) turned
from green to red.
The third one is, I hope, a better overview.
The main stem, as well as becoming red for the bottom approx. 40%, is
also not
really round; it appears to have vertically-running ridges.
Total height of most mature specimen is about 60 cms.

http://www.flickr.com/photos/27310121@N08/?saved=1

Thanks in advance.[/quote]

It looks like an epilobium - probably Epilobium parviflorum http://tinyurl.com/4k6z6e. Annual, tiresome but easily removed.