Neptunia plena are a member of the Mimosoideae subfamily of Fabaceae family(1).
When I first encountered this yellow 'Mimosa', I was thrilled to have discovered an exotic species. Obviously, I was wrong assuming that it is a Mimosa. However I was not entirely wrong, at least, as it is indeed a cousin of Mimosa, from Mimoseae tribe(2). Some even name it Mimosa plena, M. punctata, M. adenanthera, or M. lycopodioides as well as many other names (3) .
It is after a long time then I realized that it is actually Neptunia plena 假含羞草 , also known as Aquatic-Dead-and-awake. Thanks to 1001 Garden Plants in Singapore, 2nd Edition. It puzzled me though : who would plant it as a garden plant in the first place. No doubt that the flower is somehow pretty, but not till that extend of bringing it home ! I wouldn’t , for sure.
Originated from the tropical America : Texas, Costa Rica, Honduras, Nicaragua, West Indies, French Guiana, Guyana, Suriname, Venezuela, Ecuador, Peru, Paraguay (2). It is now naturalized in tropical Asia (2), becoming a noxious weed, invasive and hard to eradicate.
Neptunia seems to be water-loving, perennial herbaceous shrub (3). Leaves evenly bipinnate, mildly sensitive to physical touch; glabrous stem; the flower is actinomorphic, numerous stamens, globorous ; pods dehisces when matured (4).