Ludwigia flowers & Baench Atlas

I have (or had) a lot of Ludwigia growing out of my tank (and down the back)
and it had developed flowers. Can anyone shed any more light on how to
identify the various Ludwigia varieties? Can hybrids of Ludwigia be formed
by cross-pollination or are they done by grafting or other methods?
What are the identifying features of Ludwigia glandulosa? Baench refers to
leaves arranged in a star pattern on the stem however the picture shows
only a pair of leaves at each node (standard for Ludwigia right?) The
Baench photo shows very red leaves but we don't know if this is characteristic
of L glandulosa. The description of the flower matches the flowers on my 
Ludwigia however there is also a photo of a yellow flower identified as 
L. repens which also matches. Are there distinguishing features on the 
flowers?  Thanks.

Steve in Vancouver BC