Justus Heurnius (a Latinised form of Van Heurne), was the son of Johannes Heurnius, professor of medicine at the University of Leiden, and his wife Christina Beyers. Justus entered the University of Leiden in July 1602 to study medicine and qualified as Doctor of Medicine in April 1611. He set off immediately on extended travels in France and England, during which he decided to qualify himself for missionary work in India. From December 1615 he studied theology at the Groningsche Hoogeschool (University of Groningen). After completing his studies in 1618 he wrote a memorandum in which he urged the Dutch East India Company to promote the Christian religion in those parts of the East Indies under their control. He became a minister in Kalslagen in April 1620, but as a result of his memorandum the presbytery of Amsterdam sent him to do missionary work in the East. He sailed in the Gouda in January 1624.
On its way to the East the ship called at Table Bay in April 1624 to take in supplies, though there was no settlement there as yet. Heurnius went ashore, where he described and made simple but recognisable drawings of ten types of plants. Arriving at Batavia (now Jakarta, Indonesia) in July 1624 he sent the drawings to his elder brother Otto, who had succeeded his father as professor of medicine at Leiden. Otto passed them on to Johannes B. van Stapel (or Stapelius), who included them, with Heurnius's descriptions in Latin, in an annotated edition of the botanical works of Theophrastus which he was preparing. The book was eventually published by van Stapel's father in 1644 under the title Theophrasti Eresii de historia plantarum.... The plants drawn and described by Heurnius included Haemanthus coccineus (blood flower), Cotyledon orbiculata (plakkie), Stapelia variegata (carrion flower), Kniphofia uvaria (red-hot poker), and two species of Oxalis (sorrel). His descriptions are clearly based on the living plants, as they included details such as the colours of the flowers and the time of flowering. They show that he was an observant person and had a fair knowledge of botany. According to the botanists C. Linnaeus and C.P. Thunberg* he was the first person who made a botanically useful collection of plants in southern Africa. Though few in number, his drawings were unique and excited strong interest among European botanists. The largely South African genus of small succulent plants, Huernia (a misspelling of Heurnia; Fam. Asclepiadaceae) was named after him by Robert Brown*.
Heurnius remained at Batavia as a minister for eight years, during which time he compiled a Dutch-Latin-Chinese dictionary and translated some religious texts into Chinese. In May 1832 he was sent to the Coromandel coast of India, but was back at Batavia in September. From March 1633 he served on various islands of the East Indian archipelago, until he left for the Netherlands in December 1638. From 1840 until his death he was a clergyman in the small town of Wijk by Duurstede. In 1646 he published a revised and improved edition of Jan van Hasel's Malay translation of the Gospels of Luke and John. He also translated a number of the psalms into Malay and prepared a (Dutch?) translation of the Acts of the Apostles.