Experiments were conducted to seek understanding of the adaptive characters which contribute to the success of the twining tropical legume Siratro (Macroptilium atropurpureum) as a component of sown pastures in the seasonally-dry tropics and subtropics of north-eastern Australia. Siratro was grown both in the field and in controlled environments and was subjected to sequences of water stress. Measurements were made of plant growth, tissue water relations, leaf gas exchange, and leaf movement.Water stress reduced growth of Siratro by its effect on the daily integral of assimilation and on leaf shedding. Stressed plants also partitioned less assimilate to leaves and more to roots.Siratro postponed the effects of water shortage by maintaining its leaf water potential greater than -20 bar, while grasses grown in swards with it were about -60 to -80 bar. The tissues of Siratro were not able to tolerate leaf water potential less than about -24 bar. It made only slight changes in either its water relations or its stomatal response when exposed to a variety of water stress treatments both in the field and in controlled environments.Siratro has three main mechanisms by which it prevents leaf water deficits from developing and by which it maintains relatively high leaf water status.Good stomatal control over water loss, the stomata closing at relatively high water potential and in response to falling atmospheric humidity independent of leaf water potential.Paraheliotropic leaf movements which operate to minimize the radiation received by the leaves and which can result in them being 8-10°C cooler than leaves held horizontal during water stress. The lower temperature reduces the vapour pressure gradient between leaf and air, and hence reduces water loss.Shedding leaves which reduces the amount of transpiring tissue. As the level of water stress increases, the older leaves are shed, and the new leaves are smaller, thicker, dark green and hairy. If stress continues, whole branches die until only the crown remains, most of which is under the soil surface.While these mechanisms allow Siratro to avoid severe leaf water deficits in many circumstances, its inability to withstand leaf water potentials less than -24 bar makes it vulnerable in extended periods of drought. It is possible that increasing the ability of Siratro to tolerate water deficits could increase its survival and expand its utility.