Water caltrop, or water chestnut. Lythraceae, Myrtales. Trapa natans L. is an annual herbaceous sediment-rooted hydrophyte that develops a buoyant leaf rosette at the water surface.

Stem submerged, slender (diameter, 1–6 mm), thread-like, reaching from the bottom of the water body to its surface, length 50–200 (500) cm, stem unbranched or branched with 2–14 branches, each terminating in a buoyant leaf rosette (diameter, 15–50 cm) at the water surface. Leaves dimorphic. Submerged leaves opposite, sessile, linear, entire, caducous (20–30 mm in length, 2–2.5 mm in width; Nedukha & Kitten, 2012), replaced by unbranched adventitious roots and paired or whorled, pinnately branched, adventitious structures emerging at leaf scars (diversely referred to as leaves or roots in anatomical studies). Buoyant leaves alternate in terminal rosettes, leathery, with blades rhombic to deltoid (c. long as wide: 4–5 (6.5) cm long; 3.5–6 (8) cm wide), bases broadly cuneate or nearly truncate, entire, margins ± irregularly toothed distally, glossy dark green and glabrous adaxially, light green and glabrous or pubescent abaxially. Rosette leaf petiole up to 21 cm in length, c. 1–4 times as long as blade (leaf including petiole 7–29 cm long), pubescent, often inflated halfway into an ellipsoidal swelling.

Flowers bisexual, actinomorphic (radially symmetric), solitary in rosette leaf axils, flowering at the water surface, peduncles pubescent. Sepals 4, valvate, lanceolate to slim triangular, stiff, externally keeled and usually felted hairy, persistent as 2, 3 or 4 hardened, thorn-like horns in fruit. Petals 4, white to lilac or pink, obovate, 7–10 mm long, caducous. Stamens 4, antesepalous, anthers introrse, versatile. Ovary semi-inferior, 2-locular; ovules anatropous, pendulous, with axile placentation, one per locule, only one developed after anthesis. Pistil 1. Style, subulate. Stigma capitate, caducous. Fruit a single-seeded drupe, turbinate to short-rhombic, (0-)2-4-spined, spines broadly conic, recurved, flat triangular or horizontal, ascending, spine apex with sharp, retrorse barbs, fruit apex with dome-shaped or rounded to tetragonal crown, seldom crownless, crown apex a tuft of hairs or conic beak, exocarp ephemeral, succulent, endocarp blackish-brown, stony (Caspar & Krausch, 1981; Chen et al., 2007; Cook, 1990; Muenscher, 1944; Tutin et al., 1968).

Trapa natans was first described by Linné (1753). The generic name ‘Trapa’ originates from calcitrapa, the Latin term for caltrop, an iron instrument equipped with spikes that was used in warfare as an area denial weapon, which resembles the distinctively horned water caltrop fruits. The specific name ‘natans’ is based on the Latin word for floating, which depicts the buoyant life form of the water caltrop (Borojevic, 2009; Brown, 1880). Since its scientific description, extensive efforts have been made to adequately integrate the water caltrop into the taxonomic system (see Section 10.1). After a long-standing debate, Trapa is now a member of the Lythraceae family, classified in the order Myrtales (Graham et al., 1998, 2005; Sun et al., 2020; The Angiosperm Phylogeny Group, 1998). Despite ongoing confusion regarding species delimitation (see Section 10.1), the World Checklist of Vascular Plants (WCVP, 2023) currently accepts Trapa as a genus that encompasses eight species, namely T. assamica Wójcicki, T. hankensis Pshenn, T. hyrcana Woronow, T. incisa Siebold & Zucc., T. kashmirensis Wójcicki, T. kozhevnikoviorum Pshenn, T. natans L. and T. nedoluzhkoi Pshenn (synonyms in Table S1). Among these species, T. natans is the only species recognized as polymorphic, consisting of eight varieties, namely T. natans var. natans, T. natans var. bispinosa (Roxb.) Makino, T. natans var. pseudincisa (Nakai) ined., T. natans var. quadricaudata (Glück) B. Y. Ding & X. F. Jin, T. natans var. magnicorona (Z. T. Xiong) B. Y. Ding & X. F. Jin, T. natans var. africana Brenan, T. natans var. japonica Nakai and T. natans var. quadrispinosa (Roxb.) Makino. Several homo- and heterotypic synonyms have been classified into the currently accepted scientific variety names (Table S2; WFO, 2024). The accepted species and varieties vary in their vegetative features, such as leaf shape and size, as well as reproductive structures, such as flower colour, fruit shape and size, beak shape and height, crown shape and size, and horn number, position, size and shape (Chen et al., 2007; Ding & Jin, 2020; Wójcicki, 2001). While commonly referred to as (European) water chestnut or water caltrop, Trapa is known by various English names such as bat nut, devil nut or bull's head. Additionally, diverse country-dependent vernacular names have been documented (Borojevic, 2009; Graham, 2013; Lim, 2013).

菱角，或菱角。Lythraceae， 桃金娘。Trapa natans L. 是一种一年生草本沉积物根系的水生植物，在水面上形成浮力的叶莲座丛。

茎浸没，细长（直径，1-6 毫米），线状，从水体底部延伸到其表面，长度 50-200 （500） 厘米，茎不分枝或分枝，有 2-14 个分枝，每个分枝终止于水面的浮力叶莲座（直径，15-50 厘米）。叶二态性。淹没的叶对生，无柄，线形，完整，圆生（长20-30毫米，宽2-2.5毫米;Nedukha & Kitten， 2012），被未分枝的不定根所取代，成对或轮生，羽状分枝，在叶疤处出现不定结构（在解剖学研究中被不同地称为叶子或根）。浮力的叶子在顶生莲座丛中交替出现，革质，叶片呈菱形至三角状（c. 长至宽：4-5 （6.5） 厘米长;3.5-6 （8） 厘米宽），基部宽楔形或近截断，整个，边缘远端±不规则的齿状，有光泽的深绿色，近轴无毛，浅绿色，无毛或短柔毛。莲座叶叶柄长可达 21 厘米，约为叶片的 1-4 倍（叶包括叶柄长 7-29 厘米），有短柔毛，经常膨胀到一半，形成椭圆形肿胀。

花两性，放线形（径向对称），单生于莲座叶腋，在水面开花，花梗短柔毛。萼片 4 个，瓣状，披针形至细三角形，坚硬，外部有龙骨，通常有毛毡，果实中持续存在 2、3 或 4 个硬化的荆棘状角。花瓣4片，白色至淡紫色或粉红色，倒卵形，长7-10毫米，花纹。雄蕊 4，前花序，花药内生，多用途。子房半下，2房;胚珠背离，下垂，有轴生胎盘，每个小叶一个，开花后只有一个发育。雌蕊 1.样式，subulate。柱头状，生硬。果实单籽核果，鼻甲至短菱形，（0-）2-4刺，刺宽圆锥形，反曲，平三角形或水平，上升，刺尖有尖锐的倒刺，果实顶端有圆顶形或圆形至四方冠，很少有无冠，冠顶一簇毛或圆锥形喙，外果皮短暂，多肉，内果皮黑褐色，石质（Caspar & Krausch，1981;Chen et al.， 2007;Cook，1990 年;Muenscher，1944 年;Tutin等人，1968 年）。

Trapa natans 最早由 Linné （1753） 描述。通用名称“Trapa”源自拉丁语 calcitrapa，铁蒺藜是一种配备有尖刺的铁器，在战争中用作区域拒止武器，类似于独特的有角的水蒺藜果实。“natans”的具体名称基于拉丁语中的漂浮词，它描述了水蒺藜的浮力生命形式（Borojevic，2009 年; Brown， 1880）。自其科学描述以来，已经做出了广泛的努力，以将水铁蒺藜充分整合到分类系统中（见第 10.1 节）。经过长期的争论，Trapa 现在是 Lythraceae 家族的一员，按 Myrtales 顺序分类（Graham 等人，1998,2005; Sun等人，2020 年;被子植物系统发育组，1998 年）。尽管对物种界定一直存在混淆（见第10.1节），但世界维管植物清单（WCVP）（2023）目前接受特拉帕作为一个包含八个物种的属，即T. assamica Wójcicki，T. hankensis Pshenn，T. hyrcana Woronow，T. incisa Siebold & Zucc.，T. kashmirensis Wójcicki，T. kozhevnikoviorum Pshenn，T. natans L. 和T. nedoluzhkoi Pshenn （表 S1 中的同义词）。在这些物种中，T. natans 是唯一被认定为多态性的物种，由 8 个变种组成，即 T. natans var. natans， T. natans var. bispinosa （Roxb.）Makino， T. natans var. pseudincisa （Nakai） ined.， T. 角菜变种 quadricaudata （Glück） B. Y. Ding & X. F. Jin， T. natans var. magnicorona （Z. T. Xiong） B. Y. Ding & X. F. Jin， T. natans var. africana Brenan， T. natans var. japonica Nakai 和 T. natans var. quadrispinosa （Roxb.）牧野。几个同型和异型同义词已被归类到目前公认的科学品种名称中（表 S2;WFO，2024 年）。公认的物种和品种的营养特征各不相同，如叶形和大小，以及生殖结构，如花色、果实形状和大小、喙形和高度、冠状和大小，以及角的数量、位置、大小和形状（Chen等，2007;Ding & Jin， 2020;Wójcicki，2001 年）。 虽然通常被称为（欧洲）马蹄或水铁蒺，但特拉帕有各种英文名称，如蝙蝠坚果、魔鬼坚果或牛头。此外，还记录了不同的依赖于国家的方言名称（Borojevic，2009 年;Graham，2013 年;Lim，2013 年）。