Ludwigia

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Ludwigia

Description

Slender herbs, erect or creeping and rooting at the nodes, to large shrubs. Leaves alternate or opposite, mostly entire. Stipules absent or reduced, deltoid. Flowers borne singly, clustered, or arranged in an inflorescence. Sepals 3-7, persistent after anthesis. Petals as many as the sepals or absent, caducous, yellow or white, with contorted aestivation. Stamens as many as or twice as many as the sepals, or flowers very rarely with an intermediate number of stamens; Ovary with a number of cells equal to the number of sepals, very rarely more; Capsule irregularly dehiscent, or by a terminal pore, or by flaps separating from the valve-like top. Seeds rounded or elongate, the raphe usually easily visible and in some sections equal or nearly equal in size to the body of the seed.

Distribution

Asia-Tropical: all over the world: present
According to my synopsis () 75 spp., all over the world; in Malesia 6 spp.; one of which is certainly introduced.

Taxonomy

I have divided the genus into 17 sections, the largest of which (sect. Myrtocarpus) is neotropical. They are often shrubby with large, 4- or 5-merous flowers, dimerous stamens, prominently 4- or 5-ribbed capsules, free seeds and pollen grains shed in tetrads. They appear phylogenetically central in the genus. In Malesia this section is represented by an introduced weed, L. peruviana. Close to this section are one African (sect. Africana) and one American section (sect. Macrocarpori) with terete capsules. Following these is a series of small Old World sections which have the stamens reduced to one whorl; in one African section flowers are 3-merous. L. hyssopifolia forms a monotypic section unique in having two kinds of seeds, those in the lower part of the capsule uniseriate and embedded in the endocarp, those in the upper part pluriseriate and free, while pollen grains are single, Other sections, not represented in Malesia, have all of the seeds loosely embedded in powdery endocarp. The structure of the seed is important in the discrimination of sections.
The second major line of the genus consists of species in which the seeds are embedded in coherent chucks of woody endocarp which render the capsule a tough unit from which it is difficult to separate the seeds. The two sections belonging to this line have basically 5-merous flowers and pollen shed singly. Through the disentangling of these relationships it appears that the number of stamens is not decisive for dividing the genus into two genera as this would go across relationships and lead to heterogeneous assemblages of species.
Each Malesian species belongs to a different section and being so small in number it seems not useful to give descriptions of these sections; I refer to my revision (1963).
The cradle of the genus is probably South America with an important secondary centre of evolution in Africa. It is one of the most primitive genera in the family.

Notes

It has appeared that seeds retain viability in the herbarium in unpoisoned, not too old specimens; flowering plants can thus be raised from fruiting herbarium specimens.

Citation

HARA - in J. Jap. Bot. 1953: 289
LINNÉ - in Sp. Pl. 1753: 118
RAVEN - in Reinwardtia. 1963: 327
LINNE - in Sp. Pl. 1753: 388
FAWCETT - in J. Bot. 1926: 10
MUNZ - in Bull. Torr. Bot. CI. 1944: 152
MUNZ - in Darwiniana. 1952: 179
BACK. - in Trop. Natuur. 1914: 59
A. & R. FERNANDES - in Garcia de Orta. 1957: 109