Ludwigia Linnaeus (1753: 118)

Luo, Yike & Xie, Lei, 2023, A checklist of Onagraceae in the Pan-Himalaya region, Phytotaxa 597 (4), pp. 245-268 : 262-263

4. Ludwigia Linnaeus (1753: 118) ,

as “ Ludvigia ”.

Lectotype (designated by Britton & Brown 1913):— Ludwigia alternifolia Linnaeus (1753: 118) .

= Jussiaea L. Lectotype (designated by Hitchcock & Green 1929):

— Jussiaea repens Linnaeus (1753: 388) = Ludwigia adscendens ( Linnaeus 1767: 69) Hara (1953: 291) .

Herbs annual or perennial in Pan-Himalaya. Stems glabrous to puberulent, erect to prostrate and then often rooting at the nodes. Leaves alternate or rarely opposite, petiolate; usually entire. Flowers hermaphroditic, actinomorphic, in upper leaf axils or in spikes; sepals (3–)4–5(–7), persistent after anthesis; petals (3–)4–5(–7) or absent, yellow or white, entire; stamens as many as sepals, or twice as many as the sepals; ovary with as many locules as sepals; stigma capitate, entire or irregularly lobed. Capsule dehiscent irregularly or by a terminal pore or by flaps separating from the valvelike apex. Seeds numerous, each locule has one to several rows, sometimes embedded in endocarp. Chromosome numbers: 2n = 16, 32, and 48 ( Wagner et al. 2007).

Ludwigia is a pantropical genus of 82 species (87 taxa) currently divided into 23 sections in the world ( Wagner et al. 2007). In the Pan-Himalaya region, there are six species representing six sections. Ludwiga and Jussiaea have been previously divided into two genera depending on whether having two whorls of stamens ( Linnaeus 1753). Brenan (1953a) proposed that the two genera be combined as Jussiaea . Hara (1953) agreed with this point. However, he corrected Brenan (1953a) that Ludwigia had a precedence over Jussiaea . Later taxonomists accepted Hara’s (1953) view ( Raven 1963). Molecular evidence also supported merging the two genera ( Liu et al. 2017).

Key to species of Ludwigia in the Pan-Himalaya region

1 Stamen as same as sepal .....................................................................................................................................................................2

- Stamen twice as same as sepal ...........................................................................................................................................................4

2 Seeds embedded in endocarp; pollen in monads...................................................................................................4.2. L. epilobioides

- Seeds free; pollen in tetrads................................................................................................................................................................3

3 Seeds 0.3–0.5 mm, pluriseriate per locule; capsule 3–15 mm long ............................................................................4.5. L. perennis

- Seeds 0.5–0.6 mm, one row per locule; capsule 1.2–2.2 cm long..............................................................................4.6. L. prostrata

4 Petal white; leaves broadly oblong-elliptical; sepal 5 ............................................................................................ 4.1. L. adscendens

- Petal yellow; leaves lanceolate to subovate; sepal 4 ..........................................................................................................................5

5 Young stem and inflorescence minutely puberulous; petal 2–3 mm; capsule subterete; seeds in inflated upper capsule in 2 or more rows per locule, free, in lower capsule in one row per locule, embedded in endocarp......................................... 4.3. L. hyssopifolia

- Stem with densely spreading pubescent; petal 4–14 mm; pollen shed in tetrads; capsule 8 darker ribs; seeds in 2 or more rows per locule, free ................................................................................................................................................................ 4.4. L. octovalvis