雨豆树株高可达20m，冬季时会大量落叶。二回羽状复叶互生，羽片2～5对；小叶2～8对，小叶对生，歪卵状长椭圆形或略圆形，长达4.5cm，表面滑泽，背有绒毛。头状花序于枝端腋出，总梗长10～12.5cm，花冠长1.7cm，淡黄色，被丝状长绒毛；雄蕊20枚，花丝细长，前端淡红色，基部合生甚短。木质荚果长15～20cm，宽1.7～2.5cm，扁平或略圆柱形。
雨豆树对生叶的开合现象受到温度、光照和蒸腾作用等因素的影响。在白天时，在温度高、光照强和蒸腾作用强的情况下，反而会使雨豆树对生叶展开的角度减小；而在温度低、光照弱、蒸腾作用小的情况下，雨豆树对生叶的展开角度会增大；在温度过高，光照强度极弱的情况下，雨豆树对生叶则出现接近闭合状态。
光照对雨豆树叶片和茎的开合现象起着关键的作用，光照的强弱会影响雨豆树的叶片和茎的开合角度。白天，随着太阳高度地改变，叶子的开合角度也会随之改变，使叶子的开合角度在早上、中午和下午呈现不同状况。
植物体受到单一方向的外界刺激而引起的定向运动，称为向性运动[2]。雨豆树体内的激素，控制着叶子、茎、花的形成，且它跟大部分的植物一样对光照的反应都是正向光性的。当光照射到雨豆树的叶片的正面的茎时，生长素就移向背光的叶片背面，而背光的叶片背面的雨豆树就比受光的叶片正面长得快。结果叶片背面的茎就会向生长慢的正面的茎弯曲，即向光源的一侧弯曲。
夜晚时雨豆树的对生叶会闭合状态，植物学家称这种现象为“睡眠运动”。睡眠运动可以减小植物在夜间时热量和水分的散失，也可以保护花朵中温嫩的部分，避免受到低温的伤害。
雨豆树 - 所属科简介
雨豆树含羞草科（Mimosaceae）
描述：Mimosaceae含羞草科，双子叶植物，亦有人将其置于豆科中作为一个亚科，约56属，2800种，产全世界热带、亚热带及温带地区，中国连引入栽培的有17属，约63种，主产南部和西南部。乔木或灌木，很少草本；叶为二回羽状复叶，很少一回羽状复叶，叶柄及叶轴上常具腺体；花小，两性或杂性，辐射对称，排成穗状花序、总状花序或头状花序；萼管状，5齿裂，裂片镊合状排列，很少覆瓦状排列；花瓣镊合状排列，分离或合生成一短管；雄蕊通常多数或与花冠裂片同数或为其倍数，分离或合生成管；子房上位；果为荚果。中国有：Acacia，
Adenanthera，Albizia，Mimosa,Entada，Pithecellobium等属。
分布量世界：56属，2800种；中国：17属，63种
Albizia saman is a species of flowering tree in the pea family, Fabaceae. It is often placed in the genus Samanea, which by yet other authors is subsumed in Albizia entirely.
Saman is a wide-canopied tree with a large symmetrical crown. It usually reaches a height of 25 m (82 ft) and a diameter of 40 m (130 ft). The leaves fold in rainy weather and in the evening, hence the name Rain Tree and 5 o'clock Tree (Pukul Lima) in Malay. Several lineages of this tree are available e.g. with reddish pink and creamish golden colored flowers.
During his 1799-1804 travels in the Americas, Alexander von Humboldt encountered a giant Saman tree near Maracay (Venezuela). He measured the circumference of the parasol-shaped crown at 576 ft (about 180.8 m), its diameter at around 190 ft (about 59.6 m), on a trunk at 9 ft (about 2.8 m) in diameter and reaching just 60 ft (nearly 19 m) in height. Humboldt mentioned that the tree was reported to have changed little since the Spanish colonization of Venezuela; he estimated the Saman to be as old as the famous Canary Islands Dragon Tree (Dracaena draco) of Icod de los Vinos on Tenerife.
The tree, called Samán del Guère (transcribed Zamang del Guayre by von Humboldt) still stands today and is a Venezuelan national treasure. Just like the dragon tree on Tenerife, the age of the Saman in Venezuela is rather indeterminate. As von Humboldt's report makes clear, according to local tradition it would be older than 500 years today, which is rather outstanding by the genus' standards. It is certain however than the tree is quite more than 200 years old today. But it is one exceptional individual; even the well-learned von Humboldt could not believe it was actually the same species as the Saman trees he knew from the greenhouses at Schönbrunn Castle.
