Hemigraphis glaucescens
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The plant appeared to be rather popular in Vietnam as a medicinal herb. It was found cultivated in Singapore, though very rare. In the local plant pictorial guide (Plants in Tropical Cities) published in 2014 (page 456), this plant was named as Justicia gendarussa. However, it is unlikely to be Justicia gendarussa. The stem of Justicia gendarussa is dark maroon in colour and its flower is quite different from this plant.