Rotala indica is a species of flowering plant in the loosestrife family known by the common name Indian toothcup. It is native to Southeast Asia. This aquatic plant is best known as a popular aquarium plant and as a weed of rice fields. It is known as an introduced species and a weed in rice-growing regions in Congo, Italy, Portugal, and California and Louisiana in the United States.
The stems of the plant grow up to 30 or 40 centimeters long. Leaves are decussate, arranged oppositely in perpendicular pairs along the stems. The leaves are oval with thick, whitish, cartilaginous margins and measure up to 2 centimeters long. Flowers occur in leaf axils singly or in short, spikelike inflorescences. Each has four triangular sepals and four tiny pink petals.
(From Wikipedia on 3.2.14)

Amphibious annuals; stem erect or decumbent to 35 cm long, 4-angled or terete, rooting at the nodes. Leaves decussate, sessile or subsessile, 0.5-1.6 x 0.4-1 cm, obovate-spathulate or suborbicular, base cuneate or obtuse, margins cartilaginous, apex acute, obtuse or emarginate. Bracts dimorphic, leaf like on major branches, much smaller elliptic-oblong on flowering branches which ultimately look like bracteate spikes. Bracteoles 2, 1.5-2 mm long, linear to lanceolate. Flowers sessile, axillary, solitary. Calyx tube pink or red, turning brown, 1.5-2.5 mm long, campanulate; lobes 4, c.1 mm long, triangular, appendages absent. Petals 4, pink, c. 0.5 mm long, elliptic. Stamens 4 or some times fewer, filaments inserted at about the middle of calyx tube. Ovary c. 1 mm long, ellipsoidal; style to 1 mm long; stigma capitate. Capsule c. 2 mm long, ellipsoid, 2-valved. Seeds hemispherical.
Flowering and fruiting: July-December
Waterlogged areas and rock crevices
Tropical Asia; naturalised in Europe, Africa and America
Unknown herb-4 from Assam-Jan 2014 : Attachments (5). 5 posts by 3 authors.
Attached images are unknown  herb from Assam. Please ID the plant.  
Date :30.12.2013
Location: Assam
Family : ?
Genus & species :??
Habitat: Grows wild on rice field
Habit :Herb    

Some species of Rotala i think 🙂

Rotalasome species in India & eFI with some keys

The five photographs of Rotala from Assam is seemingly look like that of the following two species. Please confirm by observing the features of the respective species given below. 1. Rotala rosea (Poiret)C.D.K. Cook, if the flowers are 5-merous or  2. Rotala indica (Willd.) Koehne, if the flowers are 4-merous

It is Rotala indica (Willd.) Koehne

SK141OCT10-2016:ID : 17 posts by 5 authors. Attachments (3)
Sharing some pictures for ID shot at Kurintar Nepal on 29 December 2013 at 900 ft.

Lovely ground cover.

Euphorbia prostrata Aiton (accepted name) ???

This does not look like any Euphorbia to me, more close ups would have helped.

Yes …, Where is the cyathium?

Any idea about the family?

Rotala sp. of Lythraceae

Rotala indica (Willd.) Koehne ???

Does not matches with images at Rotala indica 

Six Rotala listed in Nepal and it looks the nearest !

I found a site with images of Rotala indica which it seems to match with your images.: