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