Wall.
Description
Exacum teres is a slender annual gentian of the eastern Himalaya and adjacent Indo-Burmese region, documented by GBIF (about 125 occurrences) from India, Nepal, Bangladesh, China, Myanmar and Bhutan. It grows in moist, open habitats of the warm-temperate and subtropical belt, including grassy banks, terraced field margins, forest clearings and damp roadside slopes, typically below the main forest line. The illustrated material was gathered in the valley of the Buri Gandaki at Kholabensi in Nepal at about 2100 m in October, a montane locality near the upper edge of its altitudinal range, which extends from the lowland hills up into the cooler middle hills.
The plant is an erect, glabrous herb with a quadrangular, often narrowly winged stem (the epithet teres referring to its terete-cylindric aspect) that branches above. The leaves are opposite, sessile to subsessile, ovate to lanceolate, entire, and conspicuously three- to five-nerved from the base. The flowers are borne in a lax terminal and axillary cyme; each has a four-lobed calyx with keeled, narrowly winged segments, and a rotate to broadly campanulate corolla that is blue to violet with a deeper throat. The four stamens bear large yellow anthers that open by terminal pores, a diagnostic feature of Exacum. The bilocular ovary matures into a small, septicidal capsule containing numerous minute, reticulate seeds.
Exacum is a palaeotropical genus centred on Sri Lanka, southern India and Madagascar, with E. teres representing the small Himalayan element of the lineage. As a widespread annual of disturbed and semi-natural grassland it is not considered threatened. Its poricidal anthers and blue corollas indicate buzz-pollination by bees, and several relatives are of horticultural interest as the cultivated Persian violets.
Author: Wall. · Sources: POWO (Kew), GBIF, Flora of China, Flora of Bhutan, Annotated Checklist of the Flowering Plants of Nepal, Klackenberg's monograph of Exacum (Gentianaceae)