Alternanthera


Alternanthera is a genus of flowering plants in the family Amaranthaceae. It is a widespread genus with most species occurring in the tropical Americas, and others in Asia, Africa, and Australia. Plants of the genus may be known generally as joyweeds, or Joseph's coat. Several species are notorious noxious weeds.

Description

These are annual or perennial herbs or subshrubs. While some of the better-known species are aquatic plants, most are terrestrial. They take many forms, from prostrate to erect to floating. The leaves are oppositely arranged. The inflorescence is a spike or a rounded head occurring in the leaf axils or the ends of branches. The flowers have 5 tepals. There are 3 to 5 stamens which are fused into a rim at the bases, and 5 pseudostaminodes, appendages between the stamens that are not true staminodes. The fruit is a utricle containing one seed.
The genus Alternanthera contains both terrestrial and aquatic species. The photosynthetic pathway varies in this genus: Some species undergo C3 carbon fixation, one clade of 17 species has acquired the C4 pathway, and yet other species have an intermediate C3-C4 pathway.

Species

Image:Alternanthera sessilis W2 IMG 3422.jpg|thumb|right|250px|Alternanthera sessilis
It is not yet clear how many species belong in the genus. Estimates range between 80 and 200. Plants of [the World Online] accepts 106 species.
As of April 2024, Plants of the World Online accepts the following species:

Formerly placed here

Ecology

Many species have been reported as noxious weeds, including A. angustifolia, A. caracasana, A. denticulata, A. nana, A. nodiflora, A. paronychioides, A. philoxeroides, A. sessilis, A. tenella, and A. triandra. The most important species is alligator weed, a South American aquatic plant that has spread to other continents. It is a weed of many kinds of agricultural crops, it is an invasive species that degrades native habitat, and its dense mats of vegetation clog waterways, slowing shipping and increasing flooding. Alternanthera plants are known to produce allelopathic compounds that injure other plants, including crops.
Biological pest control agents now in use to reduce alligator weed infestations include the alligator weed flea beetle, the alligator weed thrips, and the alligator weed stem borer.

Uses

A. philoxeroides and A. sessilis are eaten as vegetables in parts of Asia.
Some Alternanthera are used as ornamental plants.