Hygrophila madurensis (N.P. Balakr. & Subram.) Karthik. & Moorthy: An overlooked endemic species of Tamil Nadu, India

INTRODUCTION The family Acanthaceae is positioned under the order Lamiales and belong to the core class Euasterids I of Core Eudicots (Chase & Reveal 2009). According to the recent estimate (Karthikeyan et al. 2009) 593 Acanthaceae taxa (475 species and 118 varieties) are present in India. The genus Hygrophila R.Br. belongs to the tribe Ruellieae of family Acanthaceae (Scotland & Vollessen 2000) and comprises about 100 species (Hu & Daniel 2011). India is known to have 18 species (Karthikeyan et al. 2009, Sunojkumar & Prasad 2014), of these H. madurensis and H. thymus are endemic to Tamil Nadu (Singh et al. 2015, Kottaimuthu et al. 2018). During the course of our recent studies on the wetland plants of Madurai District, we have collected an interesting species of Acanthaceae that is characterized by distinctly pedicellate flowers, pedunculate cymes and linear–oblong capsules. Critical studies with pertinent literature; it is identified as Hygrophila madurensis (N. P. Balakr. & Subram.) Karthik. & Moorthy (Balakrishnan & Subramanyam 1963, Balakrishnan 1988, Raja et al. 2015). A perusal of literature revealed that this species is listed as critically endangered and endemic to Tamil Nadu, found along the foothills of the Eastern Ghats (Balakrishnan 1988, Nayar 1996, Walter & Gillett 1998, Reddy et al. 2006, Arisdason 2011). However, this species was not included in Acanthaceae of Eastern Ghats (Pullaiah et al. 2011). A detailed description, photographs, associated species and threat status are provided for easy identification and conservation of this little known endemic species.