The plant shown in the photo above (copyright Forest and Kim Starr) is Ludwigia octovalvis, commonly known (along with other species in the same genus) as primrose-willow. This is a very common plant in tropical and subtropical regions around the world; indeed, it is so widespread that we have little idea where in the world it originated*. The name 'primrose-willow' derives, of course, from its combination of primrose-like flowers with willow-like leaves, but it is no close relation to either. Primrose-willows belong to the Onagraceae, the same plant family as evening primrose or fuchsias. Ludwigia octovalvis is a shrubby plant, sometimes growing up to four metres in height. Lower parts of the stem may become woody with age, but the greater part of the plant is herbaceous. It prefers to grow in damp habitats, in swampy soil or alongside streams, even rooted in ponds. The seeds are minute and easily spread by water or mixed in with other materials. They are also durable: Raven (1977) refers to the possibility of propagating Ludwigia from seeds preserved in herbarium specimens.
*Which, if one were of a panbiogeographical bent, might be taken to indicate that it has survived unchanged since the Triassic at least.
Ludwigia octovalvis may even grow as floating mats upon the surface of water. The floating roots then produce spongy, upright branches called aerophores. These have been interpreted as floatation devices, but the plant is apparently perfectly buoyant without them. It is more likely that they allow oxygen to reach the waterlogged roots. The lower part of the stem may also become covered in aerenchyma, porous tissue that also aids in the diffusion of gases. If conditions dry up and the plant becomes rooted in the ground, the aerophores disappear and the roots resume their normal rootly business.
Primrose-willows are generally toxic to humans. In the usual way, I have come across references to Ludwigia octovalvis being used folk-medicinally, mostly to help with ailments of the digestive tract such as diarrhoea and worms. A quick look through Google Scholar indicates that this has lead to a certain degree of pharmaceutical research, but so far this doesn't seem to have lead to much major commercial application. At the present point in time, the main economic impact of Ludwigia octovalvis is as a weed. It can grow mixed in with fields of crops such as rice and taro, or particularly lush patches of primrose-willow may clog up waterways. On the flipside, I did find this summary of the species that notes that, "Its yellow flowers add a splash of color to areas often devoid of colorfully flowering plants".
Raven, P. H. 1977. Onagraceae. Flora Malesiana, ser. I, 8 (2): 98–113.