Botanical plate of Boswellia sacra.
Frankincense is produced by several species of the genus Boswellia. All are trees or shrubs.
The finest and most aromatic gum-resin comes from Boswellia sacra (also known as Boswellia thruifera), a small tree up to 5m high with 1 or several trunks covered with peeling, papery bark.
The leaves are crowded towards the tips of the thick twigs. Each leaf is divided into 6–8 pairs of oblong leaflets increasing in size towards the top of the leaf which is tipped by a single, largest leaflet. The leaflets all have distinctive, wavy margins and are hairy, very densely so on the underside.
The flowers are also borne at the tips of the twigs. They are loosely grouped into long, slender spikes, have five white petals and a central disc which turns from yellow to red or black as the fruit develops.
The fruit is a capsule with 3–5 longitudinal wings and opens by means of 3–5 valves, each releasing a single seed.
The genus Boswellia is named for John Boswell, uncle of James, the biographer of Samuel Johnson.
There has been, and remains, considerable confusion over the number and delimitation of the species of Boswellia. This has been compounded by: